Fix flaky pull-request job by honoring GetLastKeys arguments

[Copilot]

The pull-request workflow was failing on JwtServiceTest.ShouldGenerateFiveKeys. The failure came from JwtService.GetLastKeys(...) ignoring the requested count and from in-memory tests sharing mutable global state across classes.

• Service behavior

  • Make JwtService.GetLastKeys(int? i = null) use the caller-provided count instead of always falling back to AlgorithmsToKeep.
  • Make JwtService.GetLastKeys(int i, JwtKeyType jwtKeyType) pass through the requested i unchanged.

• Test coverage

  • Update ShouldGenerateFiveKeys to query only JWS keys and assert the expected count explicitly.
  • Add a focused assertion in store tests covering the mixed JWS/JWE case, so the default overload continues to return both key types while respecting the requested quantity.

• In-memory test isolation

  • Group in-memory store test classes into a shared xUnit collection to prevent cross-class parallel execution against the same static store.

public Task<ReadOnlyCollection<KeyMaterial>> GetLastKeys(int? i = null)
{
    return _store.GetLastKeys(i ?? _options.Value.AlgorithmsToKeep, null);
}

public Task<ReadOnlyCollection<KeyMaterial>> GetLastKeys(int i, JwtKeyType jwtKeyType)
{
    return _store.GetLastKeys(i, jwtKeyType);
}