Fix TypeForm inference in union parameters

[LouisDeconinck]

Fixes #21964

Summary

• Recognize type expressions passed to generic parameters whose type is a union containing TypeForm.
• Erase type variables only for the recognition subtype check, preserving the original context for argument checking and inference.
• Add a regression test for a TypeForm union parameter.

Tests

• PATH=".venv/bin:$PATH" .venv/bin/python runtests.py check-typeform.test
• PATH=".venv/bin:$PATH" .venv/bin/python -m mypy --config-file mypy_self_check.ini mypy/checkexpr.py
• git diff --check

AI assistance: OpenAI Codex helped investigate and implement this change; I reviewed the final diff and tests.

[github-actions]

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