fix: Mismatched signature between checker plugin API and implementation

[bzoracler]

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 and the implementation class (TypeChecker).

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

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.

[bzoracler]

Fixes #17442

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅