fix(httpx): preserve sync embed data= kwarg to avoid breaking mock-based tests

The earlier sync_httpx_client.post() call passed data=json.dumps(data),
which downstream embedding tests assert on (e.g. tests for hosted_vllm,
jina_ai, watsonx). Switching to json=data changed the kwarg name and broke
those tests. The OCI signed_body path keeps using data=signed_body and is
unaffected.

litellm/llms/custom_httpx/llm_http_handler.py

@@ -950,7 +950,7 @@ class BaseLLMHTTPHandler:
                 response = sync_httpx_client.post(
                     url=api_base,
                     headers=headers,
-                    json=data,
+                    data=json.dumps(data),
                     timeout=timeout,
                 )
         except Exception as e: