Warn on impossible-to-match message pattern in filterwarnings - #15364

Summary

This PR addresses issue #10478 by warning users when they configure a filterwarnings message containing an unescaped empty regex group (). Users often write () expecting it to match literal parentheses (e.g., ignore:The foo.bar() function is deprecated), but since the message is interpreted as a regular expression, it behaves as an empty group and silently fails to match the literal text.

What I changed

• Lightweight Scanner: Added a private scanner function _has_empty_regex_group in src/_pytest/config/__init__.py to safely detect unescaped () outside of character classes.
• Warning Emission: Added _warn_on_empty_regex_group to emit a PytestConfigWarning when escape=False, guiding the user to escape the parentheses (\(\)) or use re.escape().
• Testing: Added a test suite (TestEmptyGroupInFilterMessage) in testing/test_warnings.py validating true positives (e.g., ()) and true negatives (e.g., [()], \(\), foo(bar), foo(a|)).
• Changelog & Authors: Included the changelog file (changelog/10478.improvement.rst) and updated AUTHORS.

AI Assistance

• GitHub Copilot was used as an interactive tool to assist in drafting the regex scanner logic and parameterizing edge-case tests. All changes were rigorously reviewed, tested locally, and validated by the author.

Closes #10478