Preloaded file case insensitive name collision throws uncaught exception

I found a related issue while working on #15245

**Version of emscripten/emsdk:**

> emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 5.0.2-git (83e1f4c1829557af926eac29f66231d158bc3250)
clang version 23.0.0git (https://github.com/emscripten-core/llvm-project.git 7d5622f7917815d224b780309432ffe4729e4852)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/bin

**Failing command line in full:**

```shell
./emcc -g test/hello_world_file.cpp -o hello.html --preload-file test/hello_world_file.txt -s CASE_INSENSITIVE_FS=1 --preload-file test/hELlo_wORLd_fILe.tXt
```
This compiles (hELlo_wORLd_fILe.tXt contents do not matter much, a slight variation of hello_world_file.txt was used) but, when run with

```shell
./emrun hello.html
```
The result is a page that hangs on a spinning logo and 'all downloads complete'. The console reports the following error:
```
Uncaught (in promise) ErrnoError:
ErrnoError http://localhost:6931/hello.js:2089
mknod http://localhost:6931/hello.js:2650
create http://localhost:6931/hello.js:2691
createDataFile http://localhost:6931/hello.js:3464
FS_createDataFile http://localhost:6931/hello.js:2015
processPackageData http://localhost:6931/hello.js:179
runWithFS http://localhost:6931/hello.js:192
hello.js:2089:11
still waiting on run dependencies: hello.js:976:18
runDependencyWatcher http://localhost:6931/hello.js:976
dependency: fp /test/hello_world_file.txt hello.js:978:16
runDependencyWatcher http://localhost:6931/hello.js:978
dependency: datafile_hello.data hello.js:978:16
runDependencyWatcher http://localhost:6931/hello.js:978
(end of list) hello.js:981:16
runDependencyWatcher http://localhost:6931/hello.js:981
```
**Diagnosis**

It seems that the bug occurs after the first file (hELlo_wORLd_fILe.tXt) is loaded. When loading the second file, the function mayCreate (libfs.js:395-405) will be called:
```javascript
mayCreate(dir, name) { // checking for /test/hello_world_file.txt
if (!FS.isDir(dir.mode)) {
return {{{ cDefs.ENOTDIR }}};
}
try {
var node = FS.lookupNode(dir, name); // this respects CASE_INSENSITIVE_FS; hELlo_wORLd_fILe.tXt is the same as hello_world_file.txt

return {{{ cDefs.EEXIST }}}; //thus, this is returned (20 in the compiled code)
} catch (e) {
}
return FS.nodePermissions(dir, 'wx');
}
```
This is then passed up to mknod (libfs.js:700-703), which throws the error
```javascript
var errCode = FS.mayCreate(parent, name); //this returns cDefs.EEXIST, aka 20
if (errCode) {
throw new FS.ErrnoError(errCode); //and thrown
}
```

**Proposed Solutions**

It should be relatively simple to add a check for cDefs.EEXISTS inside a \#if CASE_INSENSITIVE_FS block. There are several possibilities:
1. Overwrite the Old File - Remove the old node (after other checks), then replace it. This seems to be in line with desired behavior in #15245, so may solve both bugs at once.
2. Keep Both - This would mean the namespace collision gets handled like a hash collision. Results are almost the same as Overwrite the Old File, except this uses more memory (which #15245 wished to avoid).
3. Skip the New File - A catch could be put somewhere along the path (createDataFile), which either swallows that error or does something to notify the user. When two files of the exact same (case sensitive) name are uploaded, this is what seems to happen (one is silently dropped)

I'd be happy to provide a PR with a fix for this.

**Full link command and output with `-v` appended:**

> ./emcc -g test/hello_world_file.cpp -o hello.html --preload-file test/hello_world_file.txt -s CASE_INSENSITIVE_FS=1 --preload-file test/hELlo_wORLd_fILe.tXt -v
>
> /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/bin/clang -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -g -v -c test/hello_world_file.cpp -o /tmp/emscripten_temp_0tijcxf2/hello_world_file.o
clang version 23.0.0git (https://github.com/emscripten-core/llvm-project.git 7d5622f7917815d224b780309432ffe4729e4852)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/bin
(in-process)
"/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/bin/clang-23" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name hello_world_file.cpp -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -fvisibility=hidden -debug-info-kind=constructor -dwarf-version=4 -debugger-tuning=gdb -fdebug-compilation-dir=/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten -v -fcoverage-compilation-dir=/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten -resource-dir /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/lib/clang/23 -D EMSCRIPTEN -isysroot /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot -internal-isystem /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/lib/clang/23/include -internal-isystem /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include -fdeprecated-macro -ferror-limit 19 -fmessage-length=157 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_0tijcxf2/hello_world_file.o -x c++ test/hello_world_file.cpp
clang -cc1 version 23.0.0git based upon LLVM 23.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include/fakesdl
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include/compat
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include/c++/v1
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/lib/clang/23/include
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/include
End of search list.
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/bin/clang --version
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/tools/file_packager hello.data --from-emcc --preload test/hello_world_file.txt test/hELlo_wORLd_fILe.tXt
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/bin/wasm-ld -o hello.wasm /tmp/tmpewl8knd4libemscripten_js_symbols.so --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=_emscripten_stack_alloc --export=__wasm_call_ctors --export=_emscripten_stack_restore --export=strerror --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --export-table -z stack-size=65536 --no-growable-memory --initial-heap=16777216 --no-entry --stack-first --table-base=1 /tmp/emscripten_temp_0tijcxf2/hello_world_file.o -L/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/cache/sysroot/lib/wasm32-emscripten -L/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/src/lib -lGL-getprocaddr -lal -lhtml5 -lstubs-debug -lnoexit -lc-debug -ldlmalloc-debug -lcompiler_rt -lc++-debug-noexcept -lc++abi-debug-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr
/mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/llvm-project/build/bin/llvm-objcopy hello.wasm hello.wasm '--remove-section=llvm.func_attr.annotate.*' --remove-section=producers
/home/linuxbrew/.linuxbrew/bin/node /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/tools/compiler.mjs -
/home/linuxbrew/.linuxbrew/bin/node /mnt/bigSSD/Master/School/3_OSU/Quarter_15/CS464/emscripten/tools/preprocessor.mjs - shell.html

Riproduci il problema con i comandi emcc ed emrun forniti usando CASE_INSENSITIVE_FS=1. Leggi libfs.js nella sezione relativa a mayCreate e mknod, quindi segui createDataFile e processPackageData per determinare come debba essere gestita la collisione. Il lavoro è concluso quando il preload non lascia più un’eccezione non gestita né una dipendenza di esecuzione in sospeso, e il comportamento di collisione scelto è coperto da un test di regressione.

Scritto dal modello di indicizzazione a partire dal testo della issue.