stubtest flags PEP 661 `sentinel` defaults, but not `object()` markers

Bug Report

I was eager to try PEP661 support on master, but found what might be an oversight in stubtest. It flags sentinel usage against the stub's parameter type, while it does allow object-style marker objects. Since PEP661's sentinel is now the canonical way to write such a marker, I'd expect it to be allowed in such places:

To Reproduce

# foo.py
from typing_extensions import sentinel
_MISSING = sentinel("_MISSING")


def f(x=_MISSING):
    pass

# foo.pyi
def f(x: int = ...) -> None: ...

python -m mypy.stubtest foo

Expected Behavior

No errors, the same as with _MISSING = object():

Success: no issues found in 1 module

Actual Behavior

error: foo.f is inconsistent, runtime parameter "x" has a default value of type typing_extensions.sentinel, which is incompatible with stub parameter type int
Stub: in file foo.pyi:1
def (x: int =)
Runtime: in file foo.py:5
def (x=_MISSING)

Found 1 error (checked 1 module)

I get the same on Python 3.15 beta.

As described above, adding the sentinel to the stub's type (x: int | _MISSING = ...) isn't an option.

The exemption seems to be in _verify_arg_default_value:

# Avoid false positives for marker objects
and type(runtime_arg.default) is not object

It could be adjusted to include sentinel the same way.

My Environment

• Mypy version used: master at 8d30064580c4273542f56ab2e9ed6a17caeea8ac (2.4.0+dev)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.14.7 (typing_extensions 4.16.0)

stubtest の _verify_arg_default_value チェックから始め、提供されている foo.py と foo.pyi の再現ケースを使って python -m mypy.stubtest foo を実行します。typing_extensions.sentinel のデフォルト値が object() マーカーとして扱われ、コマンドがエラーを報告しない一方で、互換性のない通常のデフォルト値は引き続き検出されることを確認します。