Add back in process_stream() changes removed by develop rebase

Author: tgasser-nv

nemoguardrails/rails/llm/buffer.py

@@ -114,7 +114,9 @@ def format_chunks(self, chunks: List[str]) -> str:
         ...
 
     @abstractmethod
-    async def process_stream(self, streaming_handler):
+    async def process_stream(
+        self, streaming_handler
+    ) -> AsyncGenerator[ChunkBatch, None]:
         """Process streaming chunks and yield chunk batches.
 
         This is the main method that concrete buffer strategies must implement.
@@ -139,7 +141,8 @@ async def process_stream(self, streaming_handler):
             ...     print(f"Processing: {context_formatted}")
             ...     print(f"User: {user_formatted}")
         """
-        yield ChunkBatch([], [])  # pragma: no cover
+        raise NotImplementedError
+        yield
 
     async def __call__(self, streaming_handler):
         """Callable interface that delegates to process_stream.