Fix isinstance narrowing for classes with __getattr__ returning Any

[jpbeaudry]

Fixes #16590

When a class has __getattr__ returning Any and appears in a union with a protocol type, the else-branch of isinstance fails to narrow the class away.

The problem is in narrow_declared_type() in meet.py. When processing union members against the narrowed type, is_overlapping_types(A, Protocol) returns True because __getattr__ → Any structurally satisfies any protocol. Then meet_types(A, Protocol) returns A, so A leaks back into the result and the narrowing is effectively undone.

The fix: after narrowing a union member d against a protocol n, if the result is d unchanged and d is not nominally related to n (only structurally, via __getattr__), exclude it from the narrowed union.

Test added in check-isinstance.test covering both __getattr__ → Any and __getattr__ → str cases.

This is my first contribution to mypy — happy to adjust the approach if there's a better way to handle this.

[github-actions]

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

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/frame.py:14737: error: Incompatible return value type (got "DataFrame | Series", expected "DataFrame")  [return-value]

werkzeug (https://github.com/pallets/werkzeug)
+ src/werkzeug/datastructures/file_storage.py:189: error: Unused "type: ignore" comment  [unused-ignore]

[jpbeaudry]

Primer results look correct per predominantly AI analysis:

• werkzeug: type: ignore[assignment] is now unused because the narrowing improvement makes it unnecessary.
• pandas: the return type DataFrame | Series was previously narrowed incorrectly; mypy now correctly reports the mismatch.

Both appear to be cases where the fix improves narrowing accuracy rather than introducing false positives.