Go: implement ListModels in Volcengine driver (#14702)

### What problem does this PR solve?

The VolcEngine Go driver in `internal/entity/models/volcengine.go`
shipped with a
`ListModels` stub that returned `volcengine, no such method`.
`conf/models/volcengine.json`
also did not declare a `models` URL suffix, so the model picker had
nothing to call even
if the method body were filled in.

A tenant who configured Volcengine (Doubao / Ark) as a provider could
not see the list of
available endpoints from the RAGFlow UI. Several other Go drivers
already implement
`ListModels` against the OpenAI-compatible `/models` endpoint (deepseek,
gitee, nvidia,
openai, siliconflow), so the interface and pattern are well-established.

This PR fills the gap.

### What this PR includes

* `conf/models/volcengine.json`: declare the `models` URL suffix
alongside the existing
  `chat`, `files`, and `embedding` entries. The Ark v3 API exposes
`https://ark.cn-beijing.volces.com/api/v3/models`, so the suffix is just
`models`.
* `internal/entity/models/volcengine.go`: replace the `ListModels` stub
with a real
implementation. Reuses the package-level `DSModelList` / `DSModel` types
that
DeepSeek, Gitee, and SiliconFlow already use to parse the
OpenAI-compatible models
  response shape.

No factory change. No interface change.

### How the driver works

* Resolves the region with a default fallback, the same way the other
VolcEngine methods
  in this driver already do.
* Builds the URL from `BaseURL[region] + URLSuffix.Models`, with
`strings.TrimSuffix` on
  the base to keep the join robust.
* Issues a `GET` with optional `Authorization: Bearer <api_key>` (the
header is omitted
when no key is configured, mirroring the existing NVIDIA `ListModels`).
* Reads the response body once, surfaces a non-200 with the upstream
status line plus
  body, and parses the JSON via the shared `DSModelList` type.
* Returns the model id list in input order. When the response includes
an `owned_by`
field, the entry is rendered as `id@owned_by`, matching the convention
used by the
  other Go drivers.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)


### How was this tested?

* `go build ./internal/entity/models/...` returns exit 0.
* `go vet ./internal/entity/models/...` is clean.
* `gofmt -l internal/entity/models/volcengine.go` is clean.
* The full method set on `VolcEngine` still matches the `ModelDriver`
interface.
* Endpoint reachability check: `GET
https://ark.cn-beijing.volces.com/api/v3/models`
returns `401 Unauthorized` without an API key, confirming the path
exists and accepts
  Bearer authentication.
* Pattern parity with DeepSeek, Gitee, NVIDIA, and SiliconFlow
`ListModels`.

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": [

internal/entity/models/volcengine.go

@@ -496,7 +496,60 @@ func (z *VolcEngine) Rerank(modelName *string, query string, documents []string,
 }
 
 func (z *VolcEngine) ListModels(apiConfig *APIConfig) ([]string, error) {
-	return nil, fmt.Errorf("%s, no such method", z.Name())
+	var region = "default"
+	if apiConfig != nil && apiConfig.Region != nil && *apiConfig.Region != "" {
+		region = *apiConfig.Region
+	}
+
+	baseURL := z.BaseURL[region]
+	if baseURL == "" {
+		baseURL = z.BaseURL["default"]
+	}
+	if baseURL == "" {
+		return nil, fmt.Errorf("volcengine: no base URL configured for region %q", region)
+	}
+
+	url := fmt.Sprintf("%s/%s", strings.TrimSuffix(baseURL, "/"), z.URLSuffix.Models)
+
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create request: %w", err)
+	}
+
+	if apiConfig != nil && apiConfig.ApiKey != nil && *apiConfig.ApiKey != "" {
+		req.Header.Set("Authorization", fmt.Sprintf("Bearer %s", *apiConfig.ApiKey))
+	}
+
+	resp, err := z.httpClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("failed to send request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read response: %w", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("VolcEngine models API error: %s, body: %s", resp.Status, string(body))
+	}
+
+	var modelList DSModelList
+	if err = json.Unmarshal(body, &modelList); err != nil {
+		return nil, fmt.Errorf("failed to parse response: %w", err)
+	}
+
+	models := make([]string, 0, len(modelList.Models))
+	for _, model := range modelList.Models {
+		modelName := model.ID
+		if model.OwnedBy != "" {
+			modelName = model.ID + "@" + model.OwnedBy
+		}
+		models = append(models, modelName)
+	}
+
+	return models, nil
 }
 
 func (z *VolcEngine) Balance(apiConfig *APIConfig) (map[string]interface{}, error) {