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/sandbox/tools.py

@@ -483,7 +483,7 @@ def ensure_sandbox_initialized(runtime: ToolRuntime[ContextT, ThreadState] | Non
             # Sandbox was released, fall through to acquire new one
 
     # Lazy acquisition: get thread_id and acquire sandbox
-    thread_id = runtime.context.get("thread_id")
+    thread_id = runtime.context.get("thread_id") if runtime.context else None
     if thread_id is None:
         raise SandboxRuntimeError("Thread ID not available in runtime context")