refactor(task): enhance UpdateWithStatus for CAS updates and add integration tests

- Updated UpdateWithStatus method to use Model().Select("*").Updates() for conditional updates, preventing GORM's INSERT fallback.
- Introduced comprehensive integration tests for UpdateWithStatus, covering scenarios for winning and losing CAS updates, as well as concurrent updates.
- Added task_cas_test.go to validate the new behavior and ensure data integrity during concurrent state transitions.

model/midjourney.go

@@ -160,8 +160,10 @@ func (midjourney *Midjourney) Update() error {
 // UpdateWithStatus performs a conditional UPDATE guarded by fromStatus (CAS).
 // Returns (true, nil) if this caller won the update, (false, nil) if
 // another process already moved the task out of fromStatus.
+// UpdateWithStatus performs a conditional UPDATE guarded by fromStatus (CAS).
+// Uses Model().Select("*").Updates() to avoid GORM Save()'s INSERT fallback.
 func (midjourney *Midjourney) UpdateWithStatus(fromStatus string) (bool, error) {
-	result := DB.Where("status = ?", fromStatus).Save(midjourney)
+	result := DB.Model(midjourney).Where("status = ?", fromStatus).Select("*").Updates(midjourney)
 	if result.Error != nil {
 		return false, result.Error
 	}