fix: add model_name to embed completion request (#15883) (#15888)

### What problem does this PR solve?

When embedding a chatbot, the API returned `"Model Name is required"`.
The embed widget now includes the assistant's `llm_id` as `model_name`
in the completion request.

### Type of change

- [x] Bug Fix

### How has this been tested?

- Created a chatbot with a default model.
- Embedded it and sent a message – the error is gone and the assistant
replies correctly.

### Related Issue

Closes #15883

Co-authored-by: RAGFlow Dev <dev@ragflow.local>
Co-authored-by: Wang Qi <wangq8@outlook.com>

api/apps/restful_apis/bot_api.py

@@ -149,6 +149,7 @@ async def chatbots_inputs(dialog_id, tenant_id=None):
             "avatar": dialog.icon,
             "prologue": dialog.prompt_config.get("prologue", ""),
             "has_tavily_key": bool(dialog.prompt_config.get("tavily_api_key", "").strip()),
+            "llm_id": dialog.llm_id or "",
         }
     )