redundant-expr does not work with TypeVar with multiple constraints

Bug Report
A TypeVar with multiple constraints can be seen as a union. However, the redundant-expr does not understand it and emits errors which are wrong.

To Reproduce

from typing import TypeVar

T = TypeVar("T", int, str)

def foo(var: T) -> T:
    return "aa" if isinstance(var, str) else 0

Expected Behavior

no error

Actual Behavior

foo.py:9:20: error: If condition is always false  [redundant-expr]
        return "aa" if isinstance(var, str) else 0
                       ^~~~~~~~~~~~~~~~~~~~
foo.py:9:20: error: If condition is always true  [redundant-expr]
        return "aa" if isinstance(var, str) else 0
                       ^~~~~~~~~~~~~~~~~~~~
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.14.1
• Mypy command-line flags: --enable-error-code redundant-expr
• Python version used: 3.13.0

Start by running the provided TypeVar example with mypy 1.14.1 and --enable-error-code redundant-expr. Trace the redundant-expr analysis for constrained TypeVars, then add a regression test showing that the isinstance check produces no error and run the relevant test suite.