Limit the size of the stat prefix in fuzz tests

Test case file:
[clusterfuzz-testcase-ext_proc_unit_test_fuzz-5199582416928768.txt](https://github.com/user-attachments/files/21553872/clusterfuzz-testcase-ext_proc_unit_test_fuzz-5199582416928768.txt)

fuzzer generated all dots 1Mb stats prefix. It needs to be limited to avoid excessive CPU cleanup of the stats store.

Steps to reproduce:

Copy downloaded test case file into the `test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_corpus/` directory and run:

```
bazel build --config=asan -c dbg test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_unit_test_fuzz
bazel-bin/test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_unit_test_fuzz test/extensions/filters/http/ext_proc/unit_test_fuzz/ext_proc_corpus/clusterfuzz-testcase-ext_proc_unit_test_fuzz-5199582416928768.txt
```