refactor(types): define OnBatchResult as PEP 695 generic type alias

Author: fcogidi

src/infermesh/types.py

@@ -678,7 +678,7 @@ def __len__(self) -> int:
 TranscriptionBatchResult: TypeAlias = BatchResult[TranscriptionResult]
 """Type alias for a batch of transcription results."""
 
-OnBatchResult: TypeAlias = Callable[[int, T | None, BaseException | None], None] | None
+type OnBatchResult[T] = Callable[[int, T | None, BaseException | None], None] | None
 """Generic callback type for per-result notifications in batch methods.
 
 Called as ``on_result(index, result, error)`` each time a single request