Fix /chat/completions to allow send only the latest message (#15197)

### What problem does this PR solve?

1. Fix /chat/completions to send only the latest message
2. Allo chat stream=False

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

docs/references/http_api_reference.md

@@ -4082,11 +4082,14 @@ The previous endpoint `POST /api/v1/chats/{chat_id}/completions` is deprecated.
   - `'content-Type: application/json'`
   - `'Authorization: Bearer <YOUR_API_KEY>'`
 - Body:
+
   - `"messages"`: `list[object]`
+  - `"question"`: `string`
   - `"stream"`: `boolean`
   - `"chat_id"`: `string` (optional)
   - `"session_id"`: `string` (optional)
   - `"llm_id"`: `string` (optional)
+  - `"pass_all_history_messages"`: `boolean` (optional)
 
 ##### Request example
 
@@ -4117,10 +4120,6 @@ curl --request POST \
           "stream": true,
           "session_id":"9fa7691cb85c11ef9c5f0242ac120005",
           "messages": [
-              {
-                  "role": "assistant",
-                  "content": "Hi! I'\''m your assistant. What can I do for you?"
-              },
               {
                   "role": "user",
                   "content": "Who are you?"
@@ -4131,8 +4130,10 @@ curl --request POST \
 
 ##### Request Parameters
 
-- `"messages"`: (*Body Parameter*), `list[object]`, *Required*
-  The conversation messages sent to the model.
+- `"messages"`: (*Body Parameter*), `list[object]`
+  The latest user message, or the conversation messages sent to the model when `pass_all_history_messages` is `true`. Either `messages` or `question` is required.
+- `"question"`: (*Body Parameter*), `string`
+  Latest user question. This is equivalent to passing `messages: [{"role": "user", "content": question}]`.
 - `"stream"`: (*Body Parameter*), `boolean`
   Indicates whether to output responses in a streaming way:
   - `true`: Enable streaming (default).
@@ -4143,6 +4144,8 @@ curl --request POST \
   Optional session ID. If `chat_id` is provided but `session_id` is omitted, a new session will be generated automatically.
 - `"llm_id"`: (*Body Parameter*), `string`
   Optional model override when a specific chat model should be used for this request.
+- `"pass_all_history_messages"`: (*Body Parameter*), `boolean`
+  When `chat_id` and `session_id` are provided, defaults to `false`, so the server uses stored session history and only the latest user message from the request. Set to `true` to replace/use the submitted full `messages` history, and overrides the stored session history.
 
 #### Response