perf: optional orjson JSON codec behind ENABLE_ORJSON (#27583)

Swap the JSON encoder/decoder used across the backend from stdlib json to
orjson when ENABLE_ORJSON is set — HTTP request bodies, JSONResponse
bodies, upstream provider responses, SSE chunks, and socket.io/Redis
payloads.

The flag defaults to off, in which case the app uses stdlib json and
engineio's codec verbatim, so default behaviour is unchanged.

- json_codec exports JSONCodec (stdlib json or the orjson codec) and
  SOCKETIO_JSON (engineio's codec or the orjson codec); call sites import
  JSONCodec and stay implementation-agnostic
- apply_orjson_http_json() is a no-op when the flag is off, leaving
  starlette's Request.json / JSONResponse.render untouched
- the orjson codec falls back to the stdlib for inputs orjson rejects
  (non-str dict keys, ints beyond 64 bits, NaN literals)
- orjson is imported only when the flag is on
- FastAPI(default_response_class=...) is deliberately not used: an
  explicit default disables the Pydantic direct-to-bytes fast path for
  response_model routes

backend/open_webui/socket/main.py

@@ -39,6 +39,7 @@ from open_webui.tasks import create_task, stop_item_tasks
 from open_webui.utils.access_control import has_permission
 from open_webui.utils.auth import get_verified_user_by_token
 from open_webui.utils.chat_id import is_saved_chat_id
+from open_webui.utils.json_codec import SOCKETIO_JSON
 from open_webui.utils.misc import get_output_text
 from open_webui.utils.redis import (
     build_sentinel_url,
@@ -70,10 +71,11 @@ if WEBSOCKET_MANAGER == 'redis':
         if sentinel_hosts
         else WEBSOCKET_REDIS_URL
     )
-    redis_manager = socketio.AsyncRedisManager(ws_redis_url, redis_options=WEBSOCKET_REDIS_OPTIONS)
+    redis_manager = socketio.AsyncRedisManager(ws_redis_url, redis_options=WEBSOCKET_REDIS_OPTIONS, json=SOCKETIO_JSON)
     sio = socketio.AsyncServer(
         cors_allowed_origins=SOCKETIO_CORS_ORIGINS,
         async_mode='asgi',
+        json=SOCKETIO_JSON,
         transports=(['websocket'] if ENABLE_WEBSOCKET_SUPPORT else ['polling']),
         allow_upgrades=ENABLE_WEBSOCKET_SUPPORT,
         always_connect=True,
@@ -87,6 +89,7 @@ else:
     sio = socketio.AsyncServer(
         cors_allowed_origins=SOCKETIO_CORS_ORIGINS,
         async_mode='asgi',
+        json=SOCKETIO_JSON,
         transports=(['websocket'] if ENABLE_WEBSOCKET_SUPPORT else ['polling']),
         allow_upgrades=ENABLE_WEBSOCKET_SUPPORT,
         always_connect=True,