fix(@angular/cli): discover migrations from installed packages when omitted by registry metadata

Private package registries such as GitHub Packages frequently strip out custom non-npm metadata properties (e.g., ng-update or schematics) from their remote API responses. Previously, this caused ng update to skip migration execution during updates from those registries, even though ng update --migrate-only worked correctly.

This commit adds a post-installation disk-fallback check in ng update: for any updated packages that were not scheduled for migrations from registry metadata, the CLI inspects node_modules/<package>/package.json on disk after installation. If an ng-update.migrations collection is found, it is automatically added to the migration queue and executed.

Closes #33717

Author: alan-agius4

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

@@ -634,7 +634,37 @@ export default class UpdateCommandModule extends CommandModule<UpdateCommandArgs
       }
     }
 
-    const migrations = plan.migrationsToRun;
+    const migrations = [...plan.migrationsToRun];
+
+    // Fallback: check installed packages on disk for migrations that were omitted
+    // from remote registry metadata (e.g., GitHub Packages strips 'ng-update' from API responses).
+    const existingMigrationPackages = new Set(migrations.map((m) => m.package));
+    for (const [packageName, targetVersion] of plan.packagesToUpdate) {
+      if (existingMigrationPackages.has(packageName)) {
+        continue;
+      }
+
+      const packageJsonPath = findPackageJson(this.context.root, packageName);
+      if (packageJsonPath) {
+        try {
+          const packageJson = JSON.parse(await fs.readFile(packageJsonPath, 'utf8'));
+          const ngUpdate = packageJson?.['ng-update'];
+          if (ngUpdate && typeof ngUpdate === 'object' && typeof ngUpdate.migrations === 'string') {
+            const installedVersion = plan.packageInfoMap.get(packageName)?.installed.version;
+            if (installedVersion) {
+              migrations.push({
+                package: packageName,
+                collection: ngUpdate.migrations,
+                from: installedVersion,
+                to: targetVersion,
+              });
+            }
+          }
+        } catch {
+          // Ignore read/parse errors for optional fallback
+        }
+      }
+    }
 
     if (migrations) {
       for (const migration of migrations) {