`typing.Concatenate` is not fully covered

[sobolevn]

Coverage reports:

I think that we need to cover these lines in tests.
PR is ready.

Linked PRs

• gh-128613: Increase typing.Concatenate coverage #128614
• [3.13] gh-128613: Increase typing.Concatenate coverage (GH-128614) #128623
• [3.12] gh-128613: Increase typing.Concatenate coverage (GH-128614) #128624