`mypy` widens multiple type parameters even when only the first one is relevant

Bug Report

When there are two type parameters to widen, mypy widens both to Any, even when the matched overloads agree on the second parameter and only actually disagree on the first.

Related to #19952

To Reproduce

https://mypy-play.net/?gist=6ab5724e788384125e85da02ae0386d3

from collections.abc import Sequence
from typing import Any, Generic, Protocol, assert_type, overload

from typing_extensions import TypeVar

S1 = TypeVar("S1")


class EA: ...


A = TypeVar("A", bound=EA, default=EA, covariant=True)
S2 = TypeVar("S2", bound=EA)
S2_contra = TypeVar("S2_contra", bound=EA, contravariant=True)


class Supports_ProtoAdd(Protocol[S2_contra, S2]):
    def _proto_add(self, other: S2_contra, /) -> "Ser2[S2, EA]": ...


class Ser2(Generic[S1, A]):
    def _proto_add(self, other: Any, /) -> "Ser2[Any, EA]":
        raise NotImplementedError

    @overload
    def __add__(
        self: Supports_ProtoAdd[S2_contra, S2],
        other: "S2_contra | Sequence[S2_contra]",
    ) -> "Ser2[S2]": ...
    @overload
    def __add__(self: "Ser2[bool, EA]", other: Sequence[Any]) -> "Ser2[int]": ...
    def __add__(self, other: Any) -> Any:
        raise NotImplementedError


def f(a2: "Ser2[Any, EA]", seq: "Sequence[Any]") -> None:
    assert_type(a2 + seq, "Ser2[Any, EA]")
    # error: Expression is of type "Ser2[Any, Any]", not "Ser2[Any, EA]"  [assert-type]

Expected Behaviour

Error free, as is the case for pyright, pyrefly and ty

Actual Behaviour

main.py:24: error: Expression is of type "Ser2[Any, Any]", not "Ser2[bool, EA]"  [assert-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment*

mypy 2.3.1 (playground)

Start by running the linked mypy-play reproduction and compare the inferred type with the assert_type expectation. Trace the overload matching and type-parameter widening logic involved in the add call, then add a regression test showing that only the disagreeing parameter widens while the shared EA parameter is preserved.