Make test suite parallel-safe by isolating console-dependent test behavior

[Copilot]

The test project was globally disabling xUnit parallelization to avoid cross-test interference. This change scopes that constraint to the single console-capturing test path so the rest of the suite can run in parallel safely.

• Parallelization scope reduction

  • Removed assembly-level DisableTestParallelization from ListingManager.Tests.
  • Added a dedicated non-parallel xUnit collection for console-output-sensitive tests.

• Targeted serialization for shared console state

  • Assigned ScanManagerTests to the new collection so only these tests are serialized.
  • Preserves isolation around Console.Out mutation without constraining unrelated tests.

• Deterministic assertion behavior

  • Replaced brittle ordered output expectation with captured output line comparison that is order-insensitive.
  • Keeps semantic validation (Missing test for 1.1, Missing test for 2.4) while removing dependence on directory enumeration order.

[CollectionDefinition(Name, DisableParallelization = true)]
public sealed class ConsoleOutputCollection
{
    public const string Name = "Console output collection";
}

[Collection(ConsoleOutputCollection.Name)]
public class ScanManagerTests : TempFileTestBase
{
    [Fact]
    public void ScanForMissingTests()
    {
        // capture Console.Out, execute scan, compare normalized lines
    }
}