Issue with `attrs` custom `__init__` defined

Bug Report
__init__ definition is ignored when generating attrs.define custom version.

To Reproduce

import attrs


@attrs.define
class A:
    x: int

    def __init__(self, x: str) -> None: ...


@attrs.define(init=False)
class B:
    x: int

    def __init__(self, x: str) -> None: ...

Expected Behavior

A(1) # error: Argument 1 to "A" has incompatible type "int"; expected "str"  [arg-type]
A("1") 
B(1) # error: Argument 1 to "B" has incompatible type "int"; expected "str"  [arg-type]
B("1")

Actual Behavior

A("1") # error: Argument 1 to "A" has incompatible type "str"; expected "int"  [arg-type]
B(1) # error: Argument 1 to "B" has incompatible type "int"; expected "str"  [arg-type]
B("1")

Your Environment

• Mypy version used: mypy 1.20.0+dev
• Mypy command-line flags: file.py --show-traceback
• Mypy configuration options from mypy.ini (and other config files): NA
• Python version used: Python 3.13.9

Notes
I think the issue comes from:
https://github.com/python/mypy/blob/a86f7e3ab0ce17b9839239951430348e69c3284b/mypy/plugins/attrs.py#L345
as it does not check whether the __init__ method exists, it only checks that init=False when attrs.define is called.

Start in mypy/plugins/attrs.py around line 345 and trace how attrs.define determines the generated constructor when a class already defines init. Reproduce the A and B examples from the issue, then add regression coverage showing that both custom constructors are respected and that the expected argument-type errors are reported.