fix: Mismatched signature between checker plugin API and implementation (#17343)

This PR changes the `fail` method's signature to be positional-only for
the first two parameters, due to a mismatch between the
[`CheckerPluginInterface`
signature](https://github.com/python/mypy/blob/8dd268ffd84ccf549b3aa9105dd35766a899b2bd/mypy/plugin.py#L242-L244)
and the implementation class
([`TypeChecker`](https://github.com/python/mypy/blob/8dd268ffd84ccf549b3aa9105dd35766a899b2bd/mypy/checker.py#L7116-L7118)).

```python
class CheckerPluginInterface:
    ...
    @AbstractMethod
    def fail(
        self, msg: str | ErrorMessage, ctx: Context, *, code: ErrorCode | None = None
    ) -> None: ...
```

```python
class TypeChecker(NodeVisitor[None], CheckerPluginInterface):
    ...
    def fail(
        self, msg: str | ErrorMessage, context: Context, *, code: ErrorCode | None = None
    ) -> None:
```

An alternative fix would be to change `TypeChecker.fail`'s parameter
name to `ctx`, but that has a greater disruption potential.

Co-authored-by: Shantanu <[email protected]>

Author: bzoracler

mypy/plugin.py

@@ -240,7 +240,7 @@ def type_context(self) -> list[Type | None]:
 
     @abstractmethod
     def fail(
-        self, msg: str | ErrorMessage, ctx: Context, *, code: ErrorCode | None = None
+        self, msg: str | ErrorMessage, ctx: Context, /, *, code: ErrorCode | None = None
     ) -> None:
         """Emit an error message at given location."""
         raise NotImplementedError