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>

web/src/interfaces/database/chat.ts

@@ -202,6 +202,7 @@ export interface IExternalChatInfo {
   title: string;
   prologue?: string;
   has_tavily_key?: boolean;
+  llm_id?: string;
 }
 
 export interface IMessage extends Message {