Added a new `CookieNamesTests` class to validate methods in the
`CookieNames` class, including scenarios for generating and
extracting cookie names and keys. Tests cover valid, invalid,
and unrelated cookie names, as well as round-trip validation
of `GetEnvelopeReceiverCookieName` and `GetEnvelopeReceiverKeyOrDefault`.
Updated `DigitalData.Auth.Tests.csproj` to include a project
reference to `DigitalData.Auth.Claims` for testing purposes.
