Make the SECRETS_KEY test actually discriminate from APP_KEY

The old test rotated app.key and expected the password to stay
readable, but Laravel's encrypter is a container singleton resolved
once — config()->set('app.key', ...) never rebuilds it, so the
rotation the test performed was invisible to any code path going
through it. Confirmed by mutation: rewiring Mailbox to Crypt::
encryptString/decryptString (genuinely APP_KEY-keyed, the exact
regression this constraint exists to forbid) left the old test green.

Replace it with two tests: a positive proof that rotating SECRETS_KEY
makes a stored password unreadable (the one that discriminates), and
a separate check that rotating APP_KEY does not, with
forgetInstance('encrypter') so that rotation is actually visible to
whatever the password accessor resolves.

Re-ran the same mutation against the new test: it fails (exit 1, the
SECRETS_KEY test reports the expected DecryptException was not
thrown), then passes again once the mutation is reverted.

tests/Feature/Mail/MailboxModelTest.php

@@ -1,6 +1,7 @@
 <?php
 
 use App\Models\Mailbox;
+use Illuminate\Contracts\Encryption\DecryptException;
 use Illuminate\Support\Facades\DB;
 
 beforeEach(function () {
@@ -16,11 +17,24 @@ it('stores the password encrypted and never in the clear', function () {
         ->and($box->fresh()->password)->toBe('sehr-geheim');
 });
 
-it('uses SECRETS_KEY, not APP_KEY — rotating APP_KEY leaves mail working', function () {
+it('is keyed to SECRETS_KEY — rotating it makes the password unreadable', function () {
+    // The POSITIVE proof, and the one that discriminates.
+    $box = Mailbox::factory()->create(['password' => 'geheim']);
+
+    config()->set('admin_access.secrets_key', 'base64:'.base64_encode(random_bytes(32)));
+
+    expect(fn () => $box->fresh()->password)
+        ->toThrow(DecryptException::class);
+});
+
+it('is NOT keyed to APP_KEY — rotating that leaves mail working', function () {
     $box = Mailbox::factory()->create(['password' => 'bleibt-lesbar']);
 
     config()->set('app.key', 'base64:'.base64_encode(random_bytes(32)));
 
+    // forgetInstance is what makes this test mean anything — see above.
+    app()->forgetInstance('encrypter');
+
     expect($box->fresh()->password)->toBe('bleibt-lesbar');
 });