Use direct cause for raises match failures

[lavaFreak]

Summary

• raise the match-failure AssertionError with the original exception as its direct cause
• add a regression test covering the direct-cause traceback wording
• add the required changelog fragment and author entry

Why

When pytest.raises(..., match=...) fails because the regex does not match, the resulting traceback currently reports the original exception as implicit context. Raising the AssertionError with from exc_val preserves the original exception as the direct cause instead, which matches the issue expectation and produces clearer output.

Closes #14389.

Testing

• env PYTHONPATH=/Users/garion/Work/pytest/src /Users/garion/Work/pytest/.venv/bin/python -m pytest /Users/garion/Work/pytest/testing/python/raises.py