Check element type comparability for tuple ordering comparisons

[PGrayCS]

Fixes #21042

When comparing tuples with ordering operators (<, >, <=, >=), mypy now verifies that the element types actually support the comparison operator.

Problem

The typeshed stubs for tuple define comparison methods using the class-level covariant TypeVar _T_co:

python
def lt(self, value: tuple[_T_co, ...], /) -> bool: ...

Due to covariance, when checking e.g. tuple[SupportsGT, ...] < tuple[object, ...], the forward __lt__ fails (correct), but the reverse tuple[object, ...].__gt__(tuple[SupportsGT, ...]) succeeds because tuple[SupportsGT, ...] is a subtype of tuple[object, ...]. At runtime this comparison would fail since object doesn't define __gt__.

Fix

After the tuple-level check_op succeeds for ordering operators, we now extract the element types from homogeneous tuples (tuple[X, ...]) and verify they support the comparison. If not, an Unsupported operand types error is reported.

Test case from the issue:

python
from typing import Any, Protocol, cast

class SupportsGT(Protocol):
def gt(self, other: Any, /) -> bool: ...

a = cast(tuple[SupportsGT, ...], ...)
b = cast(tuple[object, ...], ...)
c = a < b # now errors: Unsupported operand types for < (...)

All existing tests pass (7980 check tests, 0 regressions).

[github-actions]

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