Fix false positive when runtime_checkable() is used as a function

[Fridayai700]

Summary

When runtime_checkable(SomeProtocol) is called as a function (rather than used as a @runtime_checkable decorator), mypy incorrectly reports:

Only @runtime_checkable protocols can be used with instance and class checks

for isinstance(x, runtime_checkable(SomeProtocol)).

This is a valid pattern — for example, when you want to use isinstance() checks in tests without permanently marking the protocol as runtime-checkable in its definition.

Before:

from typing import Protocol, runtime_checkable

class P(Protocol):
    def meth(self) -> None: ...

x: object
isinstance(x, runtime_checkable(P))  # error: Only @runtime_checkable protocols...

After: No error.

Fix

check_runtime_protocol_test() now checks whether the expression passed to isinstance()/issubclass() is a direct call to runtime_checkable() (or typing_extensions.runtime_checkable/typing_extensions.runtime). If so, the protocol is treated as runtime-checkable at the point of use.

Test plan

• Added testRuntimeCheckableCalledAsFunctionInIsinstance covering both isinstance() and issubclass() with runtime_checkable() as a function call
• All 23 existing runtime protocol tests pass
• python -m pytest mypy/test/testcheck.py -k Runtime -o "addopts=" — all pass

Fixes #20787