fix(go): resolve custom base_url for empty default region (#15043)

### What problem does this PR solve?

Fixes custom `base_url` resolution when a model instance has no
configured region.

Some drivers read custom base URLs from `BaseURL[""]` when
`apiConfig.Region` is empty, while others normalize empty region to
`"default"` and read `BaseURL["default"]`. This PR adds the `"default"`
alias only for empty-region custom base URLs while preserving the
existing empty-region key.

Closes #15042

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

internal/service/model_service.go

@@ -53,9 +53,14 @@ func newModelDriverForBaseURL(driver modelModule.ModelDriver, providerName, regi
 		return driver, nil
 	}
 
-	newDriver := driver.NewInstance(map[string]string{
+	baseURLByRegion := map[string]string{
 		region: baseURL,
-	})
+	}
+	if region == "" {
+		baseURLByRegion["default"] = baseURL
+	}
+
+	newDriver := driver.NewInstance(baseURLByRegion)
 	if newDriver == nil {
 		return nil, fmt.Errorf("provider %s does not support custom base_url", providerName)
 	}

internal/service/model_service_test.go

@@ -26,7 +26,7 @@ func (s *stubModelDriver) Name() string {
 	return "stub"
 }
 
-func TestNewModelDriverForBaseURLPreservesEmptyRegion(t *testing.T) {
+func TestNewModelDriverForBaseURLAddsDefaultFallbackForEmptyRegion(t *testing.T) {
 	expected := &stubModelDriver{}
 	var gotBaseURL map[string]string
 	driver := &stubModelDriver{
@@ -46,8 +46,8 @@ func TestNewModelDriverForBaseURLPreservesEmptyRegion(t *testing.T) {
 	if gotBaseURL[""] != "http://localhost:1234" {
 		t.Fatalf("expected empty-region base URL, got %#v", gotBaseURL)
 	}
-	if _, ok := gotBaseURL["default"]; ok {
-		t.Fatalf("unexpected default region key in base URL map: %#v", gotBaseURL)
+	if gotBaseURL["default"] != "http://localhost:1234" {
+		t.Fatalf("expected default-region fallback base URL, got %#v", gotBaseURL)
 	}
 }