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/sora_gateway_handler.go

@@ -228,7 +228,7 @@ func (h *SoraGatewayHandler) ChatCompletions(c *gin.Context) {
 	var lastFailoverHeaders http.Header
 
 	for {
-		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionHash, reqModel, failedAccountIDs, "")
+		selection, err := h.gatewayService.SelectAccountWithLoadAwareness(c.Request.Context(), apiKey.GroupID, sessionHash, reqModel, failedAccountIDs, "", int64(0))
 		if err != nil {
 			reqLog.Warn("sora.account_select_failed",
 				zap.Error(err),