gh-5991: Fix segfault during finalization related to function_record (#6010)

* gh-5991: Fix segfault during finalization related to function_record

This patch was developed with assistance from  Claude Code Opus 4.6

Here's Claude's explanation of the crash mechanism and some reasoning for the difficulty to repro:

`tp_dealloc_impl` calls `cpp_function::destruct` which:
1. Calls `std::free()` on function_record string members (`name`, `doc`, `signature`)
2. Calls `arg.value.dec_ref()` on default argument values
3. Calls `delete rec` on the function_record

But it never calls `PyObject_Free(self)` or `Py_DECREF(Py_TYPE(self))`, which are
required for heap types.

During `_Py_Finalize`, final GC collects the heap types (which survive module dict
clearing via `tp_mro` self-references). This triggers a massive cascade:
`type_dealloc → property_dealloc → meth_dealloc → tp_dealloc_impl → destruct`.

At scale (~1,200+ function_records), the volume of `delete`/`free` calls corrupts
heap metadata, causing subsequent `std::free()` to receive garbage pointers → SEGV.

* Add detail::py_is_finalizing() wrapper to deduplicate version-guarded #ifdef blocks

Also fixes clang-tidy readability-implicit-bool-conversion warnings.

Made-with: Cursor

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Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>