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/google.json

@@ -11,7 +11,8 @@
   "models": [
     {
       "name": "gemini-2.5-flash",
-      "max_tokens": 1048576,
+      "content_length": 1048576,
+      "max_output": 65536,
       "model_types": [
         "chat"
       ],