feat: capture streaming token usage in DeepSeekClient.stream_chat

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/services/llm/deepseek_client.py

@@ -5,7 +5,7 @@ from the model response so that callers can dispatch tool invocations.
 """
 
 import time
-from typing import List, Dict, Optional
+from typing import List, Dict, Optional, Generator
 from loguru import logger
 import httpx
 
@@ -130,8 +130,14 @@ class DeepSeekClient(BaseLLMClient):
         max_tokens: Optional[int] = None,
         temperature: Optional[float] = None,
         **kwargs
-    ):
-        """Stream chat for the Deep Seek Client instance."""
+    ) -> Generator[str, None, Optional[Dict[str, int]]]:
+        """Stream chat for the Deep Seek Client instance.
+
+        Returns the trailing token-usage dict as the generator's return value
+        (read via StopIteration.value when manually driven with next()) when
+        the gateway sends one via stream_options.include_usage, else None.
+        """
+        usage: Optional[Dict[str, int]] = None
         try:
             payload = {
                 "model": self.config.model,
@@ -139,7 +145,8 @@ class DeepSeekClient(BaseLLMClient):
                 "max_tokens": max_tokens or self.config.max_tokens,
                 "temperature": temperature or self.config.temperature,
                 "top_p": kwargs.get("top_p", self.config.top_p),
-                "stream": True
+                "stream": True,
+                "stream_options": {"include_usage": True}
             }
 
             with self._client.stream("POST", "/chat/completions", json=payload) as response:
@@ -168,6 +175,9 @@ class DeepSeekClient(BaseLLMClient):
                             content = delta.get("content", "")
                             if content:
                                 yield content
+                        elif data.get("usage"):
+                            # Trailing usage-only chunk — no content to yield, just capture it.
+                            usage = data["usage"]
                     except json.JSONDecodeError:
                         continue
 
@@ -178,6 +188,8 @@ class DeepSeekClient(BaseLLMClient):
             logger.error(f"DeepSeek Stream调用失败: {e}")
             yield ""
 
+        return usage
+
     def get_available_models(self) -> List[str]:
         """Return available models for the Deep Seek Client instance."""
         return self.SUPPORTED_MODELS