Fix(Go): prevent global state pollution in local model connection check (#14669)

### What problem does this PR solve?

1. **Fix Global State Pollution in Local Providers (Critical Bug):** -
Resolved a severe concurrency and architecture issue in
`model_service.go`. Previously, `ListSupportedModels` would permanently
overwrite the global provider singleton with a localized URL instance
(`driver.NewInstance`). This caused cross-request contamination in
multi-tenant environments.
- Fixed `CheckProviderConnection` for local models (LM Studio, vLLM,
Ollama). It now properly creates a localized driver copy and injects the
`base_url` before testing the connection, entirely eliminating the
false-positive `missing base URL` error without polluting the global
state.
2. **Implement `VolcEngine` Embeddings:** - Fully implemented the
`Encode` method for the `volcengine` provider, enabling text embedding
capabilities for VolcEngine models.
3. **Enhance Region Validation in `SiliconFlow`:** - Added a strict
empty string check (`*apiConfig.Region != ""`) alongside the existing
`nil` check when parsing regions. This ensures that if an empty string
is passed, the system safely falls back to the `"default"` region,
preventing malformed URL requests and `unsupported protocol scheme`
errors.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)

internal/cli/user_command.go

@@ -1764,16 +1764,7 @@ func (c *RAGFlowClient) ChatToModel(cmd *Command) (ResponseIf, error) {
 
 	resp, err := c.HTTPClient.Request("POST", url, "web", nil, payload)
 	if err != nil {
-		if errors.Is(err, io.EOF) || errors.Is(err, io.ErrUnexpectedEOF) {
-			return nil, fmt.Errorf("connection closed (EOF): upstream overloaded or proxy timeout: %w", err)
-		}
-
-		var netErr net.Error
-		if errors.As(err, &netErr) && netErr.Timeout() {
-			return nil, fmt.Errorf("request timeout: model took too long to respond: %w", err)
-		}
-
-		return nil, fmt.Errorf("request failed: %w", err)
+		return nil, formatRequestError("Chat request", err)
 	}
 
 	if resp.StatusCode != 200 {
@@ -2407,3 +2398,21 @@ func (c *RAGFlowClient) RemoveChunks(cmd *Command) (ResponseIf, error) {
 	result.Duration = 0
 	return &result, nil
 }
+
+// formatRequestError Uniformly handle and format network errors in HTTP requests
+func formatRequestError(action string, err error) error {
+	if err == nil {
+		return nil
+	}
+
+	var netErr net.Error
+
+	switch {
+	case errors.Is(err, io.EOF), errors.Is(err, io.ErrUnexpectedEOF):
+		return fmt.Errorf("%s failed - connection closed (EOF): upstream overloaded or proxy timeout: %w", action, err)
+	case errors.As(err, &netErr) && netErr.Timeout():
+		return fmt.Errorf("%s failed - request timeout: server took too long to respond: %w", action, err)
+	default:
+		return fmt.Errorf("%s failed: %w", action, err)
+	}
+}