Add strict_warnings mode for internal pytest warnings

[coder-jatin-s]

Issue: #13964

This PR implements a strict warnings mode for internal pytest warnings.

Summary

• Added a boolean ini option strict_warnings and a CLI flag --strict-warnings.
• When strict warnings mode is enabled, all internal pytest warnings (pytest.PytestWarning and subclasses, including PytestCollectionWarning) are treated as errors.
• Integrated the behavior into the existing warnings plugin so it works consistently during config, collection, and runtest phases.

Implementation details

• In main.py:
  • Added an ini option:
    • strict_warnings = true|false
  • Added a CLI option:
    • --strict-warnings (in the pytest-warnings group), wired to config.option.strict_warnings.
• In warnings.py:
  • Updated catch_warnings_for_item to compute:
    strict_warnings = bool(config.getini("strict_warnings")) or bool(
    getattr(config.option, "strict_warnings", False)
    )
    if strict_warnings:
    warnings.filterwarnings("error", category=pytest.PytestWarning)