fix: Ensure explicit returns and correct async calls

Adds explicit `return None` statements in several functions across different modules to improve code clarity and prevent potential issues where functions might implicitly return None.
Corrects an async call in `Heartflow.deactivate_chat` by adding `await`.
Updates type hints in `heartflow_prompt_builder.py`.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/plugins/models/utils_model.py

@@ -689,7 +689,7 @@ class LLMRequest:
         stream_mode = request_content["stream_mode"]
         if response.status in policy["retry_codes"] or response.status in policy["abort_codes"]:
             await self._handle_error_response(response, retry_count, policy)
-            return
+            return None
 
         response.raise_for_status()
         result = {}