Issue when `self` is typed as a `Protocol`

Bug Report

mypy does not recognise typing in self properly when it comes to a Protocol in the following example.

To Reproduce

ttest.pyi:

from typing import Generic, Protocol, TypeVar, reveal_type

T = TypeVar("T")
T_CT = TypeVar("T_CT", contravariant=True)
T_CO = TypeVar("T_CO", covariant=True)

class OpsMixin(Generic[T]):
    def _add(self: OpsMixin[int], other: bool) -> OpsMixin[int]: ...

class Supports_Add(Protocol[T_CT, T_CO]):
    def _add(self, other: T_CT, /) -> T_CO: ...

class S(OpsMixin[T]):
    def __add__(self: Supports_Add[T_CT, T], other: T_CT) -> S[T]: ...

reveal_type(S[bool]() + True)  # mypy: S[bool], pyright: Error / Unknown

Expected Behavior

S[bool]() + True should be undefined.

Actual Behavior

ttest.pyi:16: note: Revealed type is "ttest1.S[builtins.bool]"

Your Environment

Mypy version used

1.18.2

Mypy command-line flags

none

Mypy configuration options from mypy.ini (and other config files)

[tool.mypy]
# Import discovery
namespace_packages = false
explicit_package_bases = false
ignore_missing_imports = true
follow_imports = "normal"
follow_imports_for_stubs = false
no_site_packages = false
no_silence_site_packages = false
# Disallow dynamic typing
disallow_any_unimported = false  # TODO
disallow_any_expr = false        # TODO
disallow_any_decorated = false   # TODO
disallow_any_explicit = false    # TODO
disallow_any_generics = false    # TODO
disallow_subclassing_any = false # TODO
# Untyped definitions and calls
disallow_untyped_calls = false     # TODO
disallow_untyped_defs = false      # TODO
disallow_incomplete_defs = false   # TODO
check_untyped_defs = true
disallow_untyped_decorators = true
# None and Optional handling
no_implicit_optional = true
strict_optional = true
# Configuring warnings
warn_redundant_casts = true
warn_unused_ignores = true
warn_no_return = true
warn_return_any = false     # TODO
warn_unreachable = false    # GH#27396
# Suppressing errors
ignore_errors = false
enable_error_code = "ignore-without-code" # same as in pandas
# Miscellaneous strictness flags
allow_untyped_globals = false
allow_redefinition = false
local_partial_types = false
implicit_reexport = false     # pyright behaves the same
strict_equality = true
# Configuring error messages
show_error_context = false
show_column_numbers = false
show_error_codes = true

Python version used

3.12.10

Start with the ttest.pyi reproducer and run it with mypy 1.18.2 on Python 3.12.10 using the configuration shown in the issue. Trace how the type checker handles self annotations involving Protocol, Generic, and add; done when Sbool + True is rejected as expected rather than revealing S[bool].