test: add rollback error condition tests

Add tests for Rollback method error paths:
- No previous image available
- Empty previous image string
- App deployment lock held
- App lock already acquired

Relates to #71

internal/service/deploy/deploy_rollback_test.go

@@ -0,0 +1,74 @@
+package deploy_test
+
+import (
+	"context"
+	"database/sql"
+	"log/slog"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"git.eeqj.de/sneak/upaas/internal/models"
+	"git.eeqj.de/sneak/upaas/internal/service/deploy"
+)
+
+func TestRollback_NoPreviousImage(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+	app := &models.App{
+		ID:              "app-rollback-1",
+		PreviousImageID: sql.NullString{},
+	}
+
+	err := svc.Rollback(context.Background(), app)
+	assert.ErrorIs(t, err, deploy.ErrNoPreviousImage)
+}
+
+func TestRollback_EmptyPreviousImage(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+	app := &models.App{
+		ID:              "app-rollback-2",
+		PreviousImageID: sql.NullString{String: "", Valid: true},
+	}
+
+	err := svc.Rollback(context.Background(), app)
+	assert.ErrorIs(t, err, deploy.ErrNoPreviousImage)
+}
+
+func TestRollback_DeploymentLocked(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+
+	// Simulate a deploy holding the lock
+	assert.True(t, svc.TryLockApp("app-rollback-3"))
+	defer svc.UnlockApp("app-rollback-3")
+
+	app := &models.App{
+		ID:              "app-rollback-3",
+		PreviousImageID: sql.NullString{String: "sha256:abc123", Valid: true},
+	}
+
+	err := svc.Rollback(context.Background(), app)
+	assert.ErrorIs(t, err, deploy.ErrDeploymentInProgress)
+}
+
+func TestRollback_LockedApp(t *testing.T) {
+	t.Parallel()
+
+	svc := deploy.NewTestService(slog.Default())
+
+	assert.True(t, svc.TryLockApp("app-rollback-4"))
+	defer svc.UnlockApp("app-rollback-4")
+
+	app := &models.App{
+		ID:              "app-rollback-4",
+		PreviousImageID: sql.NullString{String: "sha256:abc123", Valid: true},
+	}
+
+	err := svc.Rollback(context.Background(), app)
+	assert.ErrorIs(t, err, deploy.ErrDeploymentInProgress)
+}