feat(responses)!: improve responses + conversations implementations (#3810)

This PR updates the Conversation item related types and improves a
couple critical parts of the implemenation:

- it creates a streaming output item for the final assistant message
output by
  the model. until now we only added content parts and included that
  message in the final response.

- rewrites the conversation update code completely to account for items
  other than messages (tool calls, outputs, etc.)

## Test Plan

Used the test script from
https://github.com/llamastack/llama-stack-client-python/pull/281 for
this

```
TEST_API_BASE_URL=http://localhost:8321/v1 \
  pytest tests/integration/test_agent_turn_step_events.py::test_client_side_function_tool -xvs
```

llama_stack/core/conversations/conversations.py

@@ -135,7 +135,7 @@ class ConversationServiceImpl(Conversations):
             object="conversation",
         )
 
-        logger.info(f"Created conversation {conversation_id}")
+        logger.debug(f"Created conversation {conversation_id}")
         return conversation
 
     async def get_conversation(self, conversation_id: str) -> Conversation:
@@ -161,7 +161,7 @@ class ConversationServiceImpl(Conversations):
         """Delete a conversation with the given ID."""
         await self.sql_store.delete(table="openai_conversations", where={"id": conversation_id})
 
-        logger.info(f"Deleted conversation {conversation_id}")
+        logger.debug(f"Deleted conversation {conversation_id}")
         return ConversationDeletedResource(id=conversation_id)
 
     def _validate_conversation_id(self, conversation_id: str) -> None:
@@ -222,7 +222,7 @@ class ConversationServiceImpl(Conversations):
 
             created_items.append(item_dict)
 
-        logger.info(f"Created {len(created_items)} items in conversation {conversation_id}")
+        logger.debug(f"Created {len(created_items)} items in conversation {conversation_id}")
 
         # Convert created items (dicts) to proper ConversationItem types
         adapter: TypeAdapter[ConversationItem] = TypeAdapter(ConversationItem)
@@ -255,6 +255,12 @@ class ConversationServiceImpl(Conversations):
 
     async def list(self, conversation_id: str, after=NOT_GIVEN, include=NOT_GIVEN, limit=NOT_GIVEN, order=NOT_GIVEN):
         """List items in the conversation."""
+        if not conversation_id:
+            raise ValueError(f"Expected a non-empty value for `conversation_id` but received {conversation_id!r}")
+
+        # check if conversation exists
+        await self.get_conversation(conversation_id)
+
         result = await self.sql_store.fetch_all(table="conversation_items", where={"conversation_id": conversation_id})
         records = result.data
 
@@ -305,5 +311,5 @@ class ConversationServiceImpl(Conversations):
             table="conversation_items", where={"id": item_id, "conversation_id": conversation_id}
         )
 
-        logger.info(f"Deleted item {item_id} from conversation {conversation_id}")
+        logger.debug(f"Deleted item {item_id} from conversation {conversation_id}")
         return ConversationItemDeletedResource(id=item_id)