gh-148223: Avoid unnecessary NotShareableError in XIData FULL_FALLBACK

[jrfk]

• Issue: Avoid unnecessary NotShareableError creation in XIData FULL_FALLBACK #148223

Summary

Avoid unnecessary NotShareableError creation in _PyObject_GetXIData() on the FULL_FALLBACK path.

For types not present in the XIData registry, the previous code always called _get_xidata(), which raised NotShareableError before falling back to function/pickle handling. When pickle fallback succeeded, that exception was immediately discarded. This change adds a registry pre-check so unregistered types can skip _get_xidata() and go straight to the fallback path.

Follow-up to gh-148072 / GH-148125 (pickle cache).

Changes

• restructure the FULL_FALLBACK branch in Python/crossinterp.c
• probe the XIData registry with lookup_getdata() before calling _get_xidata()
• keep the existing path for registered types
• skip unnecessary exception creation for unregistered types
• preserve the previous NotShareableError behavior on total failure

Behavior

This is intended as a performance-only change.

• registered types still follow the existing path
• unregistered types use the same fallback chain as before
• on failure, the same final NotShareableError is raised as in the non-optimized path

Benchmarks

Baseline is upstream/main (after GH-148125). identity(x) round-trip, max_workers=1, Apple M2.

Data	Before	After	Speedup
list(100)	0.14ms	0.06ms	2.3x
dict(100)	0.20ms	0.08ms	2.5x
list(10000)	3.45ms	0.53ms	6.5x
dict(10000)	10.66ms	2.52ms	4.2x

Test plan

• ./python -m test test_interpreters — 172 tests passed
• ./python -m test test_concurrent_futures — 380 tests passed (including test_interpreter_pool)