[YukaSadaoka]

Description

Fixes #6207.

• Fixing a data race on a module's first import with another thread fetching internals under free threaded runtime.
• check_internals_local_exception_translator() reads and pushes onto internals.registered_exception_translators without holding exception_translator_mutex. This PR adds the mutex under Py_GIL_DISABLED as a fix.
• It locks the mutex directly rather than using with_exception_translators() because that calls get_internals() which would run the hook again.
• The hook only exists when __GLIBCXX__ isn't defined. Hence, libstdc++ builds are unaffected.

Test

• test_local_translator_registration_vs_internals_fetch keeps calling ensure_internals() from pybind11_tests on a thread while the main thread first-imports a new module, mod_concurrent_import.
• Ran test_exceptions.py locally with clang 19 + libc++ + -fsanitize=thread against a TSan-built free-threaded CPython 3.14.7.

Suggested changelog entry:

• Fixed a data race on free-threaded Python when a module's local exception translator was registered while another thread was fetching internals.

[rwgk]

@YukaSadaoka I merged master and added a19ef4f to guard the new lock to free-threaded Python 3.14.0rc1 and newer.

Older free-threaded builds use a nonrecursive mutex here, which can deadlock if an exception translator imports another pybind11 module. The guard preserves existing 3.13t behavior, including the original race, while fixing it on 3.14+. This keeps this PR moving without adding another 3.13t workaround (it'd be non-trivial); the intent is to formally drop 3.13t support before the next release.

[rwgk]

@YukaSadaoka I'm working on bit rot fixes (for some of the CI failures here) separately. I'll come back here after the fixes are merged.

[rwgk]

@YukaSadaoka I'm working on bit rot fixes (for some of the CI failures here) separately. I'll come back here after the fixes are merged.

The bit rot fixes were merged with #6209, then the new master was merged here.

[rwgk]

Approving based on manual review and review with GPT-6.1-Sol ultra.