perf: use the orjson codec for the Anthropic passthrough request body (#27810)

`passthrough_anthropic_messages` in `main.py` serializes the full request payload with stdlib `json` before sending it upstream. It is the largest single serialization on that path, since the body carries the whole conversation.

It now goes through `JSONCodec`, which selects orjson when `ENABLE_ORJSON` is set. The result is passed to aiohttp as `data=`, which encodes `str` as UTF-8 and sets `Content-Length` from the encoded bytes. The payload originates from a parsed request dict, so it holds only JSON-native types, and the serialized string is never hashed, compared or persisted.

The remaining stdlib `json` calls in this module are left alone: two are a debug log line and a fixed Ollama unload payload, and one parses an upstream error body.

With `ENABLE_ORJSON` unset, which is the default, `JSONCodec` is stdlib `json` and this call site behaves exactly as before.

backend/open_webui/main.py

@@ -229,6 +229,7 @@ from open_webui.utils.chat_variables import (
     normalize_chat_variables,
 )
 from open_webui.utils.embeddings import generate_embeddings
+from open_webui.utils.json_codec import JSONCodec
 from open_webui.utils.json_response import apply_orjson_http_json
 from open_webui.utils.logger import start_logger
 from open_webui.utils.middleware import (
@@ -1842,7 +1843,7 @@ async def passthrough_anthropic_messages(request: Request, form_data: dict, user
         response = await session.request(
             method='POST',
             url=request_url,
-            data=json.dumps(payload),
+            data=JSONCodec.dumps(payload),
             headers=headers,
             cookies=cookies,
             ssl=AIOHTTP_CLIENT_SESSION_SSL,