`not isinstance(x, cls)` inside classmethod does not narrow type

Bug Report

Inside a @classmethod, not isinstance(x, cls) does not appear to narrow the type, while isinstance(x, cls) does.

To Reproduce

This is what I wanted to write:

class Foo:
    def __init__(self, x: float) -> None:
        ...
    
    @classmethod
    def from_foo_or_float_cls(cls, x: Self | float) -> Self:
        if isinstance(x, cls):
            return x
        return cls(x)  # error: Argument 1 to "Foo" has incompatible type "Self | float"; expected "float"  [arg-type]

Further reduced:

class Foo:
    @classmethod
    def foo_1(cls, x: Self | float) -> None:
        assert isinstance(x, cls)
        reveal_type(x)  # expected: "Self"; was: "Self`0"; ✅

    @classmethod
    def foo_2(cls, x: Self | float) -> None:
        assert not isinstance(x, cls)
        reveal_type(x)  # expected: "float"; was: "Self`0" | builtins.float"; ❌

Expected Behavior

not isinstance(x, cls) should narrow the type union to remove Self.

Actual Behavior

Self is still part of the type union.

Your Environment

• Mypy version used: 1.19.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

Same behavior can be observers on mypy master with Python 3.14 (tested on the playground).

Reproduce the two reduced classmethod examples with mypy 1.19.1 or current master, then inspect the isinstance type-narrowing path for Self. Add regression coverage showing that assert not isinstance(x, cls) narrows Self | float to float, while preserving the existing positive narrowing behavior.