fix: prevent exception chunk from being passed to _process in astream

Pop exception from chunks list (like we do for the None sentinel) so
_process doesn't receive it. Guard chat_response access in ollama
post_processing with .get() for when no valid chunks arrived.

Signed-off-by: 0xCUB3 <skula@mit.edu>

Author: 0xCUB3

mellea/backends/ollama.py

@@ -596,7 +596,7 @@ async def post_processing(
         generate_log.backend = f"ollama::{self._get_ollama_model_id()}"
         generate_log.model_options = mot._model_options
         generate_log.date = datetime.datetime.now()
-        generate_log.model_output = mot._meta["chat_response"]
+        generate_log.model_output = mot._meta.get("chat_response")
         generate_log.extra = {
             "format": _format,
             "thinking": mot._model_options.get(ModelOption.THINKING, None),

mellea/core/base.py

@@ -325,8 +325,8 @@ async def astream(self) -> str:
             elif isinstance(chunks[-1], Exception):
                 # Mark as computed so post_process runs in finally block
                 self._computed = True
-                # Store exception to re-raise after cleanup
-                exception_to_raise = chunks[-1]
+                # Remove the exception from chunks so _process doesn't receive it
+                exception_to_raise = chunks.pop()
 
             for chunk in chunks:
                 assert self._process is not None