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>

scripts/serve.sh

@@ -9,6 +9,13 @@ set -e
 REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 cd "$REPO_ROOT"
 
+# ── Load environment variables from .env ──────────────────────────────────────
+if [ -f "$REPO_ROOT/.env" ]; then
+    set -a
+    source "$REPO_ROOT/.env"
+    set +a
+fi
+
 # ── Argument parsing ─────────────────────────────────────────────────────────
 
 DEV_MODE=true