Avoid crash for starred match subject tuples

[Sean-Kenneth-Doherty]

Fixes #21468.

This avoids recursively narrowing tuple-expression subject items when the subject tuple contains a starred expression. Those expressions do not have a one-to-one correspondence with the inferred TupleType items, so preserving the normal tuple assertion there can crash on subjects such as match foo, *bar:. Ordinary tuple expressions still keep the existing length assertion and recursive item narrowing.

A regression case was added to check-python310.test.

AI assistance was used in preparing this change; I am responsible for the submitted patch and follow-up review.

Verification:

• .venv/bin/python -m mypy --show-traceback --python-version 3.14 -c $'foo = 0\nbar = (1, 2)\nmatch foo, *bar:\n case _:\n pass\n'
• PATH=.venv/bin:$PATH .venv/bin/python runtests.py testMatchSequencePatternSubjectWithStarExprNoCrash
• PATH=.venv/bin:$PATH .venv/bin/python runtests.py check-python310.test
• TMPDIR=/home/sean/.cache/codex-tmp/go-build GOTMPDIR=/home/sean/.cache/codex-tmp/go-build PATH=.venv/bin:$PATH .venv/bin/pre-commit run --files mypy/checker.py test-data/unit/check-python310.test
• .venv/bin/python -m compileall -q mypy/checker.py
• .venv/bin/python -m mypy --config-file mypy_self_check.ini --no-incremental --num-workers 0 -p mypy -p mypyc
• PATH=$PWD/.venv/bin:$PATH .venv/bin/python runtests.py lint
• git diff --check

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅