fix: resolve cherry-pick compilation and test issues

- Add int64(0) param to SelectAccountWithLoadAwareness callers (signature change from channel scheduling refactor)
- Add UsageMapHook type and struct field to StreamingProcessor
- Revert Claude Max cache billing code to upstream/main (not part of channel feature)
- Revert credits overages logic to upstream/main (non-channel change)
- Remove Instructions field reference (non-channel OpenAI feature)
- Restore sora_client_handler_test.go from upstream + add channel service nil params

backend/internal/handler/gemini_v1beta_handler.go

@@ -360,7 +360,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	}
 
 	for {
-		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionKey, modelName, fs.FailedAccountIDs, "") // Gemini 不使用会话限制
+		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionKey, modelName, fs.FailedAccountIDs, "", int64(0)) // Gemini 不使用会话限制
 		if err != nil {
 			if len(fs.FailedAccountIDs) == 0 {
 				googleError(c, http.StatusServiceUnavailable, "No available Gemini accounts: "+err.Error())