fix(test): use non-installed package in dpkg/rpm Install_Unsupported tests

The tests expected Install("tree") to error because dpkg/rpm need
a .deb/.rpm file path. But "tree" is pre-installed in CI containers,
so the code short-circuits to "unchanged" and returns nil.

Use a non-existent package name instead to exercise the actual
install failure path.

dpkg/dpkg_integration_test.go

@@ -164,7 +164,7 @@ func TestIntegration_Dpkg(t *testing.T) {
 	t.Run("Install_Unsupported", func(t *testing.T) {
 		// dpkg install requires a .deb file path, not a package name
 		// This should fail gracefully
-		_, err := mgr.Install(ctx, snack.Targets("tree"))
+		_, err := mgr.Install(ctx, snack.Targets("nonexistent-pkg-xyzzy"))
 		assert.Error(t, err)
 	})
 

rpm/rpm_integration_test.go

@@ -156,7 +156,7 @@ func TestIntegration_RPM(t *testing.T) {
 
 	// --- Operations rpm doesn't support ---
 	t.Run("Install_Unsupported", func(t *testing.T) {
-		_, err := mgr.Install(ctx, snack.Targets("tree"))
+		_, err := mgr.Install(ctx, snack.Targets("nonexistent-pkg-xyzzy"))
 		assert.Error(t, err)
 	})