wasm32-wasi: opening/reading a directory as a file surfaces EBADF, not IsADirectory

I tried this code:

use std::fs::File;
use std::io::Read;

fn main() {
    let mut f = File::open("/some/dir").unwrap(); // succeeds on WASI
    let mut buf = [0u8; 16];
    println!("{:?}", f.read(&mut buf));
}

I expected to see this happen: an Err with kind() == IsADirectory, matching Unix (EISDIR) and Windows.

Instead, this happened: Err with kind() == Uncategorized, raw_os_error() == Some(8) (EBADF). WASI defers the directory check from open to the first read, and reports it as EBADF rather than EISDIR, so it falls through every arm of decode_error_kind (shared between unix/wasi since #143189) to Uncategorized.

Reproduced on both wasm32-wasip1 and wasm32-wasip2 via wasmtime 45.

Note EBADF is ambiguous (a real closed-fd error must not become IsADirectory), so this likely cannot be a decode_error_kind table entry — it probably needs to be handled contextually in the WASI open/File::open path in sys/fs/unix.rs, where std already knows a directory was being opened as a file. See uutils/coreutils#13625 for a userland workaround (wasi_normalize_open_error) doing exactly this at the call site.

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rustc --version --verbose:

rustc 1.99.0-nightly (ad3d0bc14 2026-07-31)

Reproduce the example for wasm32-wasip1 and wasm32-wasip2 via wasmtime, then inspect library/std/src/sys/fs/unix.rs and the surrounding open/File::open path. Trace how the deferred directory read error reaches decode_error_kind; done means opening a directory as a file reports IsADirectory while genuine closed-file EBADF behavior remains unchanged.