fix: add null checks for runtime.context in middlewares and tools (#1269)

Add defensive null checks before accessing runtime.context.get() to
prevent AttributeError when runtime.context is None. This affects:
- UploadsMiddleware
- MemoryMiddleware
- LoopDetectionMiddleware
- SandboxMiddleware
- sandbox tools
- setup_agent_tool
- present_file_tool
- task_tool

Also adds .env loading in serve.sh for environment variable support.

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/tools/builtins/setup_agent_tool.py

@@ -24,7 +24,7 @@ def setup_agent(
         description: One-line description of what the agent does.
     """
 
-    agent_name: str | None = runtime.context.get("agent_name")
+    agent_name: str | None = runtime.context.get("agent_name") if runtime.context else None
 
     try:
         paths = get_paths()