refactor: migrate all handlers to shared endpoint normalization middleware

- Apply InboundEndpointMiddleware to all gateway route groups
- Replace normalizedOpenAIInboundEndpoint/normalizedOpenAIUpstreamEndpoint and normalizedGatewayInboundEndpoint/normalizedGatewayUpstreamEndpoint with GetInboundEndpoint/GetUpstreamEndpoint
- Remove 4 old constants and 4 old normalization functions (-70 lines)
- Migrate existing endpoint normalization test to new API

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/handler/gemini_v1beta_handler.go

@@ -504,6 +504,8 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 
 		// 使用量记录通过有界 worker 池提交，避免请求热路径创建无界 goroutine。
 		requestPayloadHash := service.HashUsageRequestPayload(body)
+		inboundEndpoint := GetInboundEndpoint(c)
+		upstreamEndpoint := GetUpstreamEndpoint(c, account.Platform)
 		h.submitUsageRecordTask(func(ctx context.Context) {
 			if err := h.gatewayService.RecordUsageWithLongContext(ctx, &service.RecordUsageLongContextInput{
 				Result:                result,
@@ -511,6 +513,8 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 				User:                  apiKey.User,
 				Account:               account,
 				Subscription:          subscription,
+				InboundEndpoint:       inboundEndpoint,
+				UpstreamEndpoint:      upstreamEndpoint,
 				UserAgent:             userAgent,
 				IPAddress:             clientIP,
 				RequestPayloadHash:    requestPayloadHash,