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/openai_chat_completions.go

@@ -261,8 +261,8 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 				User:             apiKey.User,
 				Account:          account,
 				Subscription:     subscription,
-				InboundEndpoint:  normalizedOpenAIInboundEndpoint(c, openAIInboundEndpointChatCompletions),
-				UpstreamEndpoint: normalizedOpenAIUpstreamEndpoint(c, openAIUpstreamEndpointResponses),
+				InboundEndpoint:  GetInboundEndpoint(c),
+				UpstreamEndpoint: GetUpstreamEndpoint(c, account.Platform),
 				UserAgent:        userAgent,
 				IPAddress:        clientIP,
 				APIKeyService:    h.apiKeyService,