fix(gemini): enable model_mapping filtering for Gemini API Key accounts

Remove the special case that bypassed model-supported checks for Gemini
API Key accounts, allowing model_mapping to filter requests properly.
Add tests for multiplatform model filtering behavior.

backend/internal/service/gateway_service.go

@@ -2825,10 +2825,6 @@ func (s *GatewayService) isModelSupportedByAccount(account *Account, requestedMo
 	if account.Platform == PlatformAnthropic && account.Type != AccountTypeAPIKey {
 		requestedModel = claude.NormalizeModelID(requestedModel)
 	}
-	// Gemini API Key 账户直接透传，由上游判断模型是否支持
-	if account.Platform == PlatformGemini && account.Type == AccountTypeAPIKey {
-		return true
-	}
 	// 其他平台使用账户的模型支持检查
 	return account.IsModelSupported(requestedModel)
 }