feat: revamp DeepSeek v4 model handling

- replace legacy DeepSeek ids with the new deepseek-v4 model family\n- move thinking control to request parameters and preserve assistant reasoning content\n- switch history split to IGNORE transcript injection and map upload auth failures to 401\n- update admin defaults, API docs, samples, and tests for the new model scheme

tests/raw_stream_samples/markdown-format-example-20260405/meta.json

@@ -9,7 +9,7 @@
         "role": "user"
       }
     ],
-    "model": "deepseek-reasoner-search",
+    "model": "deepseek-v4-pro-search",
     "stream": true
   },
   "capture": {