Fix false positive for constrained NamedTuple self types

[BHUVANSH855]

Fixes #21453

This PR fixes a false positive reported for methods on generic NamedTuple classes that use constrained type variables.

Example:

from typing import Generic, NamedTuple, TypeVar

S = TypeVar("S", str, bytes)

class Result(NamedTuple, Generic[S]):
    value: S

    def get_value(self) -> S:
        return self.value

Previously mypy incorrectly reported:

The erased type of self ... is not a supertype of its class ...

The issue occurred during self-type validation for tuple-backed NamedTuple types using constrained TypeVars.

This change adds a narrow special-case in the self-type checker to avoid the false positive while preserving existing subtype semantics.

Changes included:

• Added a targeted fix in mypy/checker.py
• Added a regression test in test-data/unit/check-selftype.test

Verification:

• Reproduced the original issue locally
• Added regression coverage
• Ran python -m pytest -q mypy/test/testcheck.py -n0
• All tests passed

[github-actions]

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