Fall back to FilePersistence when libsecret is unavailable

On Linux without libsecret installed, PersistenceCreator.createPersistence()
fails because the keytar native module cannot load libsecret-1.so.0. The
usePlaintextFileOnLinux fallback in PersistenceCreator only handles
verification failures, not module load failures.

Catch the error in createPersistence() and fall back to FilePersistence
directly, ensuring the CLI works on Linux environments without libsecret
(CI, containers, headless servers).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: waldekmastykarz

src/auth/msalCachePlugin.spec.ts

@@ -51,6 +51,16 @@ describe('msalCachePlugin', () => {
     assert.notStrictEqual(persistence, undefined);
   });
 
+  it(`falls back to FilePersistence when PersistenceCreator fails`, async () => {
+    const msalExtensions = await import('@azure/msal-node-extensions');
+    sinon.stub(msalExtensions.PersistenceCreator, 'createPersistence').rejects(new Error('libsecret not available'));
+    const filePersistenceCreateStub = sinon.stub(msalExtensions.FilePersistence, 'create').resolves(mockPersistence as any);
+
+    const persistence = await msalCachePlugin.createPersistence();
+    assert(filePersistenceCreateStub.calledOnce);
+    assert.strictEqual(persistence, mockPersistence);
+  });
+
   it(`creates plugin using PersistenceCachePlugin`, async () => {
     const plugin = await msalCachePlugin.createPlugin(mockPersistence);
     assert.notStrictEqual(plugin, undefined);

src/auth/msalCachePlugin.ts

@@ -17,11 +17,19 @@ let _initPromise: Promise<{ plugin: ICachePlugin; persistence: IPersistence }> |
 
 export const msalCachePlugin = {
   async createPersistence(): Promise<IPersistence> {
-    const { DataProtectionScope, PersistenceCreator } = await import('@azure/msal-node-extensions');
-    return PersistenceCreator.createPersistence({
-      ...persistenceConfiguration,
-      dataProtectionScope: DataProtectionScope.CurrentUser
-    });
+    const { DataProtectionScope, FilePersistence, PersistenceCreator } = await import('@azure/msal-node-extensions');
+    try {
+      return await PersistenceCreator.createPersistence({
+        ...persistenceConfiguration,
+        dataProtectionScope: DataProtectionScope.CurrentUser
+      });
+    }
+    catch {
+      // PersistenceCreator fails on Linux when libsecret is not installed
+      // because the keytar native module cannot load. Fall back to an
+      // unencrypted file, which matches the usePlaintextFileOnLinux intent.
+      return FilePersistence.create(persistenceConfiguration.cachePath);
+    }
   },
 
   async createPlugin(persistence: IPersistence): Promise<ICachePlugin> {