fix(@angular/cli): address reviewer feedback for PR #33718

- Add JSDoc to `resolveFallbackMigrations` in `ng update` documenting why the post-installation disk check is needed for fallback migrations (private registries like GitHub Packages stripping custom non-npm metadata).
- Avoid cleaning up temporary dependencies during `ng add` if they are part of `BUILT_IN_SCHEMATICS`, preventing an inefficient install-uninstall-reinstall cycle.

TAG=agy
CONV=b92ba928-67b0-4f4b-a79a-ce19e5098641

Author: alan-agius4

packages/angular/cli/src/commands/add/cli.ts

@@ -261,7 +261,7 @@ export default class AddCommandModule
               if (localManifest['ng-add']?.save === false) {
                 shouldCleanUp = true;
               }
-            } else {
+            } else if (!Object.hasOwn(BUILT_IN_SCHEMATICS, result.collectionName)) {
               await this.cleanUpTemporaryDependency(result.collectionName);
               shouldCleanUp = false;
             }

packages/angular/cli/src/commands/update/cli.ts

@@ -707,6 +707,15 @@ async function readPackageManifest(manifestPath: string): Promise<PackageManifes
   }
 }
 
+/**
+ * Resolves migrations from installed package manifests on disk when they were omitted
+ * from the initial update plan.
+ *
+ * This fallback is necessary because private package registries (such as GitHub Packages)
+ * frequently strip custom non-npm metadata properties (like `ng-update`) from their remote
+ * registry API responses. By inspecting `node_modules/<package>/package.json` after installation,
+ * we ensure that any migration collections defined by the package are discovered and queued.
+ */
 export async function resolveFallbackMigrations(
   workspaceRoot: string,
   plan: UpdatePlan,