test(quality): cover routing sqlite overlay hotspots

internal/overlay/executor_test.go

@@ -151,6 +151,19 @@ func TestApplyPatchFileRejectsMissingPatch(t *testing.T) {
 	}
 }
 
+func TestApplyPatchFileRejectsInvalidPatch(t *testing.T) {
+	outputDir := t.TempDir()
+	patchPath := filepath.Join(t.TempDir(), "invalid.patch")
+	if err := os.WriteFile(patchPath, []byte("not a patch\n"), 0o644); err != nil {
+		t.Fatalf("WriteFile() error = %v", err)
+	}
+
+	err := applyPatchFile(context.Background(), outputDir, patchPath)
+	if err == nil {
+		t.Fatal("applyPatchFile() error = nil, want invalid patch failure")
+	}
+}
+
 func TestWriteMetadataIncludesSourceDirAndOverlays(t *testing.T) {
 	metadataPath := filepath.Join(t.TempDir(), metadataFileName)
 	overlays := []pack.HostOverlay{{OverlayID: "sample", PatchPath: "overlays/sample.patch"}}
@@ -208,3 +221,31 @@ func TestCopyTreeSkipsGitAndPreservesSymlink(t *testing.T) {
 		t.Fatalf("symlink target = %q, want backend/hello.txt", target)
 	}
 }
+
+func TestApplyRejectsExistingOutputDir(t *testing.T) {
+	sourceDir := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(sourceDir, "backend"), 0o755); err != nil {
+		t.Fatalf("MkdirAll() error = %v", err)
+	}
+	outputDir := filepath.Join(t.TempDir(), "existing-output")
+	if err := os.MkdirAll(outputDir, 0o755); err != nil {
+		t.Fatalf("MkdirAll(outputDir) error = %v", err)
+	}
+
+	_, err := Apply(context.Background(), ApplyRequest{
+		PackDir:   t.TempDir(),
+		SourceDir: sourceDir,
+		OutputDir: outputDir,
+		Overlays:  []pack.HostOverlay{{OverlayID: "sample", PatchPath: "overlays/sample.patch"}},
+	})
+	if err == nil || !strings.Contains(err.Error(), "already exists") {
+		t.Fatalf("Apply() error = %v, want existing output rejection", err)
+	}
+}
+
+func TestCopyFileRejectsMissingSource(t *testing.T) {
+	err := copyFile(filepath.Join(t.TempDir(), "missing.txt"), filepath.Join(t.TempDir(), "target.txt"), 0o644)
+	if err == nil {
+		t.Fatal("copyFile() error = nil, want missing source failure")
+	}
+}