Sync: keep the remembered sign-in when the provider is merely down, and say why SYNC NOW found the machine offline

tests/DodoSSH.Client.App.Tests/ShellFlowTests.cs

@@ -9702,6 +9702,42 @@ public sealed partial class ShellFlowTests : IAsyncLifetime
         resumeAttempts.ShouldBe(attempted);
     }
 
+    /// <remarks>
+    /// The machine this was found on: the identity provider restarted, discovery answered 502 for a
+    /// moment, and that was read as a refusal — so the remembered sign-in was deleted and the machine
+    /// stayed OFFLINE long after the server was back, with SYNC NOW saying nothing about why.
+    /// </remarks>
+    [Fact]
+    public async Task AProviderThatIsDown_KeepsTheSignInAndComesBackOnline()
+    {
+        await UnlockedAsync();
+        await shell.Vault!.SyncOnOpenAsync(Token);
+        await shell.LockCommand.ExecuteAsync(null);
+
+        resumeFailure = new OidcException(
+            "Discovery at 'https://id.example/.well-known/openid-configuration' returned 502.");
+
+        var relaunch = Relaunch(resume: ResumeAsync);
+        await using var _ = relaunch.ConfigureAwait(false);
+
+        await relaunch.StartAsync(Token);
+        relaunch.Passphrase = Passphrase;
+        await relaunch.UnlockCommand.ExecuteAsync(null);
+
+        await relaunch.Vault!.SyncCommand.ExecuteAsync(null);
+
+        relaunch.IsOnline.ShouldBeFalse();
+        relaunch.Vault.Status.ShouldContain("returned 502", Case.Insensitive,
+            "SYNC NOW must say why it is offline, not just that it is");
+
+        // The provider comes back, and the next pass resumes with the token it still has.
+        resumeFailure = null;
+        await relaunch.Vault.SyncOnOpenAsync(Token);
+
+        relaunch.IsOnline.ShouldBeTrue();
+        resumedWith.ShouldBe("refresh-token-1");
+    }
+
     /// <remarks>
     /// The pass that runs when the vault opens used to be skipped in the application and nowhere else: the
     /// loop is started from inside the unlock command, so the busy flag it yields to was raised by the