fix(go-models): add VolcEngine model listing suffix (#15234)

## Summary
- add the VolcEngine `models` URL suffix used by the existing Go
`ListModels` implementation
- return a clear error when the VolcEngine models suffix is missing
- add focused VolcEngine model-listing regression tests

## What changed
- Added `url_suffix.models` to `conf/models/volcengine.json`.
- Normalized the configured models suffix before building the request
URL.
- Covered config loading, successful model listing, upstream errors, and
missing suffix handling.

## Why
`VolcEngine.ListModels` already builds requests from `URLSuffix.Models`,
but the bundled VolcEngine config did not define that suffix. That left
the model-listing path unable to call the documented `/models` endpoint
from the existing provider config.

Fixes #14701

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

conf/models/volcengine.json

@@ -6,7 +6,8 @@
   "url_suffix": {
     "chat": "chat/completions",
     "files": "files",
-    "embedding": "embeddings/multimodal"
+    "embedding": "embeddings/multimodal",
+    "models": "models"
   },
   "class": "volcengine",
   "models": [
@@ -29,4 +30,4 @@
       ]
     }
   ]
-}
+}