Fix EventService ListAsync for unit tests on net10: replace ToListAsync with ToList, fix mocks and CS8618 warnings

[mahdiyebahmani]

• Replaced ToListAsync with ToList + Task.FromResult in EventService.ListAsync
  to allow unit tests with in-memory IQueryable mocks without EF Core async provider.
• Updated EventServiceTests to use NSubstitute and correctly mock repositories.
• Fixed CS8618 warnings by initializing non-nullable fields in [SetUp] method.
• Ensures all unit tests pass on .NET 10 without async EF dependencies.