[giovanamalaia]

Summary

This PR improves the error messages raised when two or more plugins attempt to register conflicting command-line options.

What I changed

As discussed in #11514, when multiple plugins register the same command-line option (e.g., --browser), the standard argparse.ArgumentError or ValueError does not provide enough context to help the user identify the source of the conflict.

This implementation:

• Tracks which plugin registers which option in _pytest/config/argparsing.py.
• Intercepts option collisions across the same or different argument groups.
• Enriches the error messages to explicitly name the conflicting plugins involved (e.g., "already added by plugins: ['plugin_a', 'plugin_b']").
• Emits a generic fallback message if the option's origin cannot be determined deterministically.
• Includes tests in testing/test_parseopt.py validating various conflict scenarios (same group, cross-group, conftest paths, entry points).

AI Assistance

• GitHub Copilot was used as an assistant to map argparse exceptions and structure test cases, with all changes reviewed, tested, and validated locally by the author.

Closes #11514