refactor(relay): improve channel locking and retry logic in RelayTask

- Enhanced the RelayTask function to utilize a locked channel when available, allowing for better reuse during retries.
- Updated error handling to ensure proper context setup for the selected channel.
- Clarified comments in ResolveOriginTask regarding channel locking and retry behavior.
- Introduced a new field in TaskRelayInfo to store the locked channel object, improving type safety and reducing import cycles.

relay/common/relay_info.go

@@ -619,6 +619,11 @@ type TaskRelayInfo struct {
 	PublicTaskID string
 
 	ConsumeQuota bool
+
+	// LockedChannel holds the full channel object when the request is bound to
+	// a specific channel (e.g., remix on origin task's channel). Stored as any
+	// to avoid an import cycle with model; callers type-assert to *model.Channel.
+	LockedChannel any
 }
 
 type TaskSubmitReq struct {