Extend `redundant-expr` to type narrowing assertions

Feature

Assert statements can be used to guide the type checker but are not checked for redundancy like casts.

Pitch

Consider this simple example, with the --enable-error-code=redundant-expr flag:

def f(x: int) -> None:
    assert x is not None           # no issue
    assert x is not None and True  # error: Left operand of "and" is always true

I think it would be useful to have mypy emit a warning for the first line.

No file or test is named. Start by locating the existing redundant-expr handling for casts and the assert type-narrowing path, then inspect the relevant tests. Done means redundant assertions such as assert x is not None are reported under --enable-error-code=redundant-expr without changing valid narrowing behavior.