Add whatsnew entry for mypy test suite improvements (#10770)

Documents the removal of all test files from mypy exclusions in PR #10768.

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Co-authored-by: Claude <noreply@anthropic.com>

Author: max-sixty

doc/whats-new.rst

@@ -77,6 +77,10 @@ Documentation
 Internal Changes
 ~~~~~~~~~~~~~~~~
 
+- All test files now have full mypy type checking enabled (``check_untyped_defs = true``),
+  improving type safety and making the test suite a better reference for type annotations.
+  (:pull:`10768`)
+  By `Maximilian Roos <https://github.com/max-sixty>`_.
 
 .. _whats-new.2025.09.0: