refactor: add String() methods to domain types, replace string() casts

internal/service/deploy/deploy_container_test.go

@@ -35,7 +35,7 @@ func TestBuildContainerOptionsUsesImageID(t *testing.T) {
 		t.Fatalf("buildContainerOptions returned error: %v", err)
 	}
 
-	if opts.Image != string(expectedImageID) {
+	if opts.Image != expectedImageID.String() {
 		t.Errorf("expected Image=%q, got %q", expectedImageID, opts.Image)
 	}