`safe-super` change breaks Mixin support

Bug Report

We want to indicate that a (super-)class must implement a specific protocol in order to inherit from a specific (sub-) class. We used to annotate self as the super class in the subclass to express this. With mypy 1.10.0 this introduces a safe-super error. Is there currently a way to express this requirement?

To Reproduce

from typing import Protocol


class HasFormValid(Protocol):
    def form_valid(self, form):
        pass


class SaveValidFormMixin:

    def form_valid(self: HasFormValid, form):
        form.save()
        super().form_valid(form)

Expected Behavior

No error: Since we never actually inherit from the HasFormValid protocol, this should be safe.

Actual Behavior

mypy 1.10.0 treats HasFormValid.form_valid as trivial and warns about its usage. In this case, it is assured that HasFormValid.form_valid cannot be actually called via super().form_valid.

mypy <1.10.0 does not raise an error here.

Your Environment

• Mypy version used: mypy 1.10.0 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12.3

Start by reproducing the provided Mixin and Protocol example with mypy 1.10.0, then trace the safe-super diagnostic and its regression tests. Add coverage for this Mixin pattern and verify that it no longer reports a false positive while existing safe-super warnings remain intact.