Incorrect note references builtins.pyi instead of the class definition

Bug Report

When reporting a "Unexpected keyword argument" error for an initializer, the note can incorrectly reference builtins.pyi instead of the class definition.

To Reproduce
https://mypy-play.net/?gist=b50a11afee5d039fb382667dee8b529f

class Foo:
    bar: str
    
def f() -> Foo:
    foo = Foo(bar="bar")
    return foo

f()

For context, I was using pydantic, but forgot to make the class in question inherit from pydantic.BaseModel. But this isn't specific to pydantic.

Expected Behavior

The note should include the class definition, not an incorrect link to builtins.pyi.
This was misleading because I thought my class was shadowing a built-in one, so I tried renaming it.

Actual Behavior

main.py:5: error: Unexpected keyword argument "bar" for "Foo"  [call-arg]
/layers/google.python.pip/pip/lib/python3.12/site-packages/mypy/typeshed/stdlib/builtins.pyi:117: note: "Foo" defined here

I believe the line referenced here is for object.__init__().

Your Environment

• Mypy version used: 1.19.1
• Mypy command-line flags: n/a
• Mypy configuration options from mypy.ini (and other config files): n/a (default for mypy playground)
• Python version used: python3.12

Start by running the Python reproducer from the issue and compare the diagnostic note for the initializer with the expected class definition reference. Trace how the error identifies the initializer and formats its source location, then confirm the fix still reports the unexpected keyword argument correctly without pointing to builtins.pyi.