fix: reset gil_safe_call_once_and_store cache on interpreter finalize

[henryiii]

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Description

When subinterpreter support is enabled, gil_safe_call_once_and_store keeps the stored value in the
state dict of the interpreter, and caches a pointer to it plus an "initialized" flag in the module.
Nothing cleared that cache when the interpreter was finalized. After
finalize_interpreter(); initialize_interpreter(); the fast path gave freed memory back to the
caller. py::register_exception uses this class, thus re-importing an embedded module which
registers an exception used the freed exception object.

The per-interpreter storage now knows its owner and resets the cached pointer and the flag from its
destructor. The interpreter destroys the storage during finalization, thus the fast path becomes
invalid at the correct time and the next call stores the value again.

New Catch2 tests in tests/test_with_catch/test_interpreter.cpp restart the interpreter and then
use the stored value. Before the fix, the callable was not called again for the new interpreter, and
the re-imported exception module had no exception attribute.

Suggested changelog entry:

• Fixed a use-after-free in py::gil_safe_call_once_and_store (and therefore in
  py::register_exception) when an embedded interpreter is finalized and then initialized again.