Fix object property value in mlx_lm.server chat completions response to match OpenAI spec

These were "chat.completions" and "chat.completions.chunk"
but should be "chat.completion" and "chat.completion.chunk"
for compatibility with clients expecting an OpenAI API.

In particular, this solves a problem in which aider 0.64.1 reports
hitting a token limit on any completion request, no matter how small,
despite apparently correct counts in the usage property.

Refer to:

https://platform.openai.com/docs/api-reference/chat/object

> object string
> The object type, which is always chat.completion.

https://platform.openai.com/docs/api-reference/chat/streaming

> object string
> The object type, which is always chat.completion.chunk.

llms/mlx_lm/SERVER.md

@@ -92,7 +92,7 @@ curl localhost:8080/v1/chat/completions \
 
 - `system_fingerprint`: A unique identifier for the system.
 
-- `object`: Any of "chat.completions", "chat.completions.chunk" (for
+- `object`: Any of "chat.completion", "chat.completion.chunk" (for
   streaming), or "text.completion".
 
 - `model`: The model repo or path (e.g. `"mlx-community/Llama-3.2-3B-Instruct-4bit"`).

llms/mlx_lm/server.py

@@ -589,9 +589,7 @@ class APIHandler(BaseHTTPRequestHandler):
 
         # Determine response type
         self.request_id = f"chatcmpl-{uuid.uuid4()}"
-        self.object_type = (
-            "chat.completions.chunk" if self.stream else "chat.completions"
-        )
+        self.object_type = "chat.completion.chunk" if self.stream else "chat.completion"
         if (
             hasattr(self.tokenizer, "apply_chat_template")
             and self.tokenizer.chat_template