Fix ambiguous failure and error headings

[Mo-Thabet]

Closes #14902.

Summary

• Include the full test node ID in failure and error headings only when multiple reports share the same heading.
• Add regression coverage for duplicate test names across modules in both fixture errors and assertion failures.

Verification

• 252 passed, 2 skipped across testing/test_terminal.py and testing/test_reports.py.
• Full testing/ suite: 4,533 passed, 157 skipped, 13 xfailed, 1 xpassed; one unrelated Windows test_pydoc failure due to a UTF-8 decoding error.
• Ruff checks, formatting, and git diff --check passed.

Prepared with AI assistance under human direction and review; the commit includes the required co-author trailer.