incorrectly reports type error for union types in property setters

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## Problem Description

MyPy extension in VS Code incorrectly reports a type error, while mypy from the command line finds no issues.

## Environment

- **VS Code Version**: 1.102.0
- **MyPy Extension**: mypy-type-checker
- **MyPy Version**: 1.20.0 (compiled: yes)
- **Python Version**: 3.12.10

## MyPy Extension Configuration

```json
{
"mypy-type-checker.path": [
"C:\\Users\\GSWI\\AppData\\Local\\Programs\\Python\\Python312\\Scripts\\mypy.exe"
]
}
```

## Code Sample

```python
class A:
_x: int = 0

@property
def x(self) -> int:
return self._x

@x.setter
def x(self, value: int | str) -> None:
self._x = int(value)

a = A()
a.x = "42" # ← VS Code MyPy extension reports error here
```

## Expected Behavior

The code should pass without errors because:
1. The setter `x` accepts both `int` and `str` (union type `int | str`)
2. MyPy from command line confirms the code is correct

## Actual Behavior

**VS Code MyPy Extension**:
```
Incompatible types in assignment (expression has type "str", variable has type "int")
```

**Command Line MyPy**:
```
> C:\Users\GSWI\AppData\Local\Programs\Python\Python312\Scripts\mypy.exe main.py
Success: no issues found in 1 source file
```

Reproduce the reported false positive in VS Code using the provided Python property-setter example, then compare the extension's behavior with the command-line mypy result. Done means the assignment to a.x accepts the int | str setter type without an erroneous diagnostic; the payload does not name an implementation file or regression test.

Written by the indexing model from the issue text.