PR #21749 mypy has ZERO production code changes - test-data annotations only, uses non-existent Python 3.15

[knoal]

Problem - HIGH

PR #21749 (Fix TypedDict closed argument version check) has ZERO production code changes. Only test-data annotations are added.

Impact

• The PR title claims a fix but no fix is applied
• The bug is NOT fixed
• Uses Python 3.15 which does not exist as a released version (latest stable is 3.13, 3.14 in pre-release)
• The test cases only add # flags: --python-version 3.15 to existing test definitions
• No regression tests for the specific bug

What was audited

MCE audit Tmypy21749 (2026-07-19) caught 4 HIGH attacks via F-C:

1. The test cases only add # flags: --python-version 3.15 to existing test definitions - there is NO assertion change, NO new expected error line, and NO behavioral test added
2. The diff shown contains ZERO production-code changes - only test-data annotations
3. Python 3.15 does not exist as a released version (latest stable is 3.13, 3.14 in pre-release)
4. The three modified tests are not regression tests for the specific bug - they predate the fix and already had expected behavior

Recommended fix

1. The author should add the actual source code change that fixes the TypedDict closed argument version check
2. Remove the # flags: --python-version 3.15 lines (use an existing version)
3. Add proper regression tests with assertion changes

Cross-references

• MCE audit: Tmypy21749 (2026-07-19), 0/4 HIT, Brier 0.6
• F-C analysis: /home/alex/.hermes/mce/fc-critiques/Tmypy21749-with-diff.json
• PR: Fix TypedDict closed argument version check #21749