fix: preserve PEP 661 sentinels in dict.get inference

[mgalore]

Fixes #21866.

dict.get() infers its default through a type variable. For a PEP 661 sentinel, constraint inference used the Sentinel fallback instance instead of its sentinel literal, producing str | sentinel rather than str | Unknown.

Preserve a sentinel last_known_value when creating a direct type-variable constraint, and add the reported regression case.

Verification:

• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python -m pytest -n0 -q mypy/test/testcheck.py::TypeCheckSuite::check-sentinels.test
• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python runtests.py (12,860 + 601 + 55 tests passed; expected skips/xfails)
• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python runtests.py lint
• .venv/bin/pre-commit run black --files mypy/constraints.py
• .venv/bin/pre-commit run ruff-check --files mypy/constraints.py

[github-actions]

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

[cdce8p]

While this appears to solve the issue, I'm not sure we should change constaints.py. We don't do that for enums either.