Fix Any identity narrowing of enum sentinels

[dannyward630]

Fixes #21667.

This prevents identity/equality narrowing from using an Any comparison target to widen a non-Any expression. In the reported sentinel pattern, x: Any still narrows to the enum literal when x is SENTINEL, but the explicitly typed SENTINEL: SentinelType no longer becomes Any in the true branch.

Tests:

• python runtests.py EnumSentinelComparedToAny
• python runtests.py check-enum.test
• python -m mypy --config-file mypy_self_check.ini mypy/checker.py

Disclosure: I used an LLM-assisted coding tool to help inspect the narrowing path and prepare this patch; I reviewed the change and tests before submitting.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spark (https://github.com/apache/spark)
- python/pyspark/sql/pandas/types.py:791: error: Unused "type: ignore" comment  [unused-ignore]
- python/pyspark/sql/pandas/types.py:792: error: Unused "type: ignore" comment  [unused-ignore]

steam.py (https://github.com/Gobot1234/steam.py)
+ steam/ext/commands/commands.py:245: error: Value of type "def [T] Greedy(cls, *args: Any, **kwargs: Any) -> Never" is not indexable  [index]

strawberry (https://github.com/strawberry-graphql/strawberry)
+ strawberry/utils/typing.py:304: error: Incompatible return value type (got "<typing special form>", expected "type")  [return-value]

pydantic (https://github.com/pydantic/pydantic)
+ pydantic/main.py:1300: error: Invalid self argument "BaseModel" to attribute function "__repr_recursion__" with type "Callable[[Representation, Any], str]"  [misc]

cloud-init (https://github.com/canonical/cloud-init)
+ tests/unittests/sources/azure/test_kvp.py:101: error: Item "None" of "HyperVKvpReportingHandler | None" has no attribute "vm_id"  [union-attr]
+ tests/unittests/sources/azure/test_kvp.py:127: error: Item "None" of "HyperVKvpReportingHandler | None" has no attribute "vm_id"  [union-attr]