Update package versions and improve null safety in tests

- Updated various package versions in `Directory.Packages.props` to the latest compatible releases, enhancing overall stability and security.
- Modified test assertions in `GrantBillingGiftCommandHandlerTests`, `RejectPaymentRequestCommandHandlerTests`, `BlockUserCommandHandlerTests`, and `DeleteUserCommandHandlerTests` to use null-safe checks, ensuring robustness against potential null reference exceptions.
- Updated PostgreSqlContainer initialization in `PnvPanelWebApplicationFactory` for improved clarity and maintainability.

backend/tests/PnvPanel.Application.Tests/Admin/Billing/GrantBillingGiftCommandHandlerTests.cs

@@ -74,7 +74,7 @@ public class GrantBillingGiftCommandHandlerTests
             .Received(1)
             .NotifyUserAsync(
                 userId,
-                Arg.Is<string>(m => m.Contains("30")),
+                Arg.Is<string>(m => m!.Contains("30")),
                 Arg.Any<string?>(),
                 Arg.Any<CancellationToken>()
             );

backend/tests/PnvPanel.Application.Tests/Admin/Billing/RejectPaymentRequestCommandHandlerTests.cs

@@ -71,7 +71,7 @@ public class RejectPaymentRequestCommandHandlerTests
             .Received(1)
             .NotifyUserAsync(
                 userId,
-                Arg.Is<string>(m => m.Contains("Платёж не найден")),
+                Arg.Is<string>(m => m!.Contains("Платёж не найден")),
                 Arg.Any<string?>(),
                 Arg.Any<CancellationToken>()
             );