Add /health endpoint to server (#5136)

Co-authored-by: openhands <openhands@all-hands.dev>

openhands/server/listen.py

@@ -94,6 +94,11 @@ app.add_middleware(
 )
 
 
+@app.get('/health')
+async def health():
+    return 'OK'
+
+
 security_scheme = HTTPBearer()