fix: handle qwen rerank error response (#15881)

### What problem does this PR solve?
Fix QWen rerank error handling so DashScope error responses without a
text attribute do not raise a secondary KeyError and hide the real
provider error.

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

rag/llm/rerank_model.py

@@ -432,7 +432,16 @@ class QWenRerank(Base):
                 log_exception(_e, resp)
             return rank, total_token_count_from_response(resp)
         else:
-            raise ValueError(f"Error calling QWenRerank model {self.model_name}: {resp.status_code} - {resp.text}")
+            try:
+                error_body = resp["text"] if isinstance(resp, dict) and "text" in resp else None
+            except Exception:
+                error_body = None
+            if not error_body:
+                try:
+                    error_body = json.dumps(dict(resp), ensure_ascii=False)
+                except Exception:
+                    error_body = str(resp)
+            raise ValueError(f"Error calling QWenRerank model {self.model_name}: {resp.status_code} - {error_body}")
 
 
 class HuggingfaceRerank(Base):