fix(storage): unify CAS persistence for inline images

- Externalize inline images in messages and run events before persistence
- Keep snapshots, transcripts, and metadata free of inline payloads
- Serialize messages after externalization to reduce peak memory usage
- Preserve live messages, URL-only images, and SDK image retrieval behavior
- Add regression tests for deduplication, redaction, storage failures, and serialization order

src/langbot/pkg/agent/runner/platform_tools.py

@@ -794,7 +794,9 @@ async def execute_platform_tool(
                         bot_name=str(getattr(getattr(bot, 'bot_entity', None), 'name', bot_id)),
                         pipeline_id=str(authorization.get('processor_id') or ''),
                         pipeline_name=str(authorization.get('processor_id') or authorization.get('runner_id') or ''),
-                        message_content=json.dumps(content, ensure_ascii=False),
+                        message_content=content
+                        if isinstance(content, (list, dict))
+                        else json.dumps(content, ensure_ascii=False),
                         session_id=str(authorization.get('conversation_id') or ''),
                         role='assistant',
                         status='error' if delivery_error else 'success',