Handle functools.Placeholder in partial

[MestreY0d4-Uninter]

Fixes #21313.

Summary

This updates the functools.partial plugin to treat functools.Placeholder as an unbound positional slot instead of a normal bound argument.

The generated partial signature now keeps placeholder-backed parameters available for later calls, while still using non-placeholder arguments for type inference.

Test plan

• PYTHONPATH=. uv run --no-project --isolated --with-requirements test-requirements.txt python -m pytest -q mypy/test/testcheck.py::TypeCheckSuite::check-functools.test
• PYTHONPATH=. uv run --no-project --isolated --with-requirements test-requirements.txt python -m mypy --config-file mypy_self_check.ini mypy/plugins/functools.py

[MestreY0d4-Uninter]

Pushed 6bc619f to address the mypy_primer INTERNAL ERROR: the partial plugin now avoids direct type-var inference on Unpack/TypeVarTuple arguments while keeping the Placeholder narrowing path.

Local validation:

• uv run --with-requirements test-requirements.txt python runtests.py check-functools.test — 31 passed
• uv run --with-requirements test-requirements.txt python runtests.py lint — passed
• Minimal TypeVarTuple partial repro no longer crashes; it reports normal type errors instead.

[github-actions]

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