fix(vertex): audit fixes for Vertex Service Account feature (#1977)

- Security: force token_uri to Google default, preventing SSRF via crafted service account JSON
- Dedup: extract shared getVertexServiceAccountAccessToken() to eliminate ~35 lines of duplication between ClaudeTokenProvider and GeminiTokenProvider
- Fix: apply model mapping + Vertex model ID normalization in forward_as_responses and forward_as_chat_completions paths
- Fix: exclude service_account from AI Studio endpoint selection (Vertex cannot serve generativelanguage.googleapis.com)
- Feature: add model restriction/mapping UI for service_account in EditAccountModal
- Dedup: extract VERTEX_LOCATION_OPTIONS to shared constants
- i18n: replace all hardcoded Chinese strings in Vertex UI with translation keys

backend/internal/service/gemini_messages_compat_service.go

@@ -515,6 +515,10 @@ func (s *GeminiMessagesCompatService) SelectAccountForAIStudioEndpoints(ctx cont
 			}
 			// Code Assist OAuth tokens often lack AI Studio scopes for models listing.
 			return 3
+		case AccountTypeServiceAccount:
+			// Vertex service accounts use aiplatform.googleapis.com, not the AI Studio
+			// endpoint (generativelanguage.googleapis.com), so they cannot serve these requests.
+			return 999
 		default:
 			return 10
 		}