feat: populate content_length and max_output from official docs (#17836)

## Summary

- Verify and populate `content_length` (context window) and `max_output`
(max generation tokens) for all **478 chat/vision models** across **47
provider configs**
- Data sourced from **official API documentation** via 12 parallel
agents + targeted web verification
- Update Go test assertions to match verified values

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

conf/models/volcengine.json

@@ -14,7 +14,8 @@
   "models": [
     {
       "name": "doubao-seed-2-0-pro-260215",
-      "max_tokens": 262144,
+      "content_length": 256000,
+      "max_output": 16384,
       "model_types": [
         "chat"
       ],