Incorrect argument names for dataclass with "private" (mangled) members

Bug Report

Mypy infers dataclass arguments incorrectly when a class has private (__name) members.

To Reproduce

from dataclasses import dataclass


@dataclass(kw_only=True)
class Data:
    __mangled: int


Data(_Data__mangled=1)  # Runs correctly, but mypy complains
Data(__mangled=1)       # Fails, but mypy approves

https://mypy-play.net/?mypy=latest&python=3.13&gist=f585904f6f5652ee86cd11d6502de13f

Expected Behavior

I expect mypy's behaviour to match the execution and requrie the _Data__mangled argument.
Additionally, I expect it to reject the __mangled argument name as unexpected.

Actual Behavior

main.py:9: error: Unexpected keyword argument "_Data__mangled" for "Data"; did you mean "__mangled"? [call-arg]
Found 1 error in 1 file (checked 1 source file)

Your Environment

I reproduced this on mypy-play.net with Python 3.13

• Mypy version used: 1.17.1
• Mypy command-line flags: default mypy-play.net settings
• Python version used: Python 3.13

Start with the reproducer in main.py and run mypy 1.17.1 against Python 3.13, comparing the accepted keyword names with runtime dataclass behavior. Trace the dataclass argument inference used for the private member and verify that _Data__mangled is accepted while __mangled is rejected.