C#: method name should be PascalCase (Please refer to implementation of backward compatibility here )

Tests in R are disproportionately harder than in other languages. 1000 tests make fail many approaches valid elsewhere (~100 tests in Python and NASM, ~30 in JS and C...).

Honestly, great Job.

good fun kata!.

Ruby 3.0 should be enabled, read this to learn how to do it

Please organize the structure of test fixture as following (although it has been mentioned in the attached link, I'm repeated here again)

describe "<This message should describe the categories of test groups inside this block>" do
  it "<A short message describing this test group>" do
    expect(...).to eq(...) #Assertions
    #Or Test.assert_equals(user_response, reference_response)
  end
end

This is free using any BigInteger type.

Improve your explanation as it is not clear why intervalList1 should to give false answer. Test basic is clearly in proper range, room is not occupied in its time interval.

        StartDate = DateTime.Parse("4/6/2019 1:02:32 PM"),
        EndDate = DateTime.Parse("4/6/2019 1:03:32 PM")

if we will confront it with list when the room is occupied

        StartDate = DateTime.Parse("3/2/2019 3:04:32 PM"),
        EndDate = DateTime.Parse("3/5/2019 5:06:32 PM")

        StartDate = DateTime.Parse("4/6/2019 1:04:32 PM"),
        EndDate = DateTime.Parse("4/6/2019 1:06:32 PM")

Why don't you block access to biginteger? Your kata will be much more interesting.