gh-153128: Preserve once warnings across filter changes

[snoopuppy582]

Summary

• keep the C implementation's "once" warning tracking in the global once registry
• make that registry independent of warning filter version changes, matching _py_warnings
• add a shared regression test for the C and pure Python implementations
• add a Library NEWS entry

The C implementation previously reused the per-module warning registry whenever one was available. That registry is deliberately cleared when the filter version changes, so adding even an unrelated filter caused a warning governed by "once" to be emitted again. The pure Python implementation stores the (text, category) key in onceregistry, which is not filter-versioned.

This change factors the key lookup/update logic from the versioned registry helper and uses it with the global once registry. "default" and "module" tracking remain versioned as before.

Fixes #153128.

Tests

• PCbuild/build.bat -p x64 -c Debug
• PCbuild/amd64/python_d.exe -m unittest -v test.test_warnings.CFilterTests.test_once_after_filter_change test.test_warnings.PyFilterTests.test_once_after_filter_change
• PCbuild/amd64/python_d.exe -m test -v test_warnings
• PCbuild/amd64/python_d.exe -X context_aware_warnings=1 -m test test_warnings
• PCbuild/amd64/python_d.exe -m test -R 3:3 test_warnings
• PCbuild/amd64/python_d.exe Tools/patchcheck/patchcheck.py

AI assistance

OpenAI Codex was used to assist with duplicate research, reproduction, root-cause analysis, the C implementation and regression test, and local verification. The code and this PR description were prepared with that assistance; this PR is submitted as a draft for review.

• Issue: warnings: adding new filter resets prior existing "once" filters #153128

[python-cla-bot]

The following commit authors need to sign the Contributor License Agreement:

• mnb0968@naver.com