feat(tests): add unit test project with 29 tests

[ZenonEl]

Summary

• Add TelegramMediaRelayBot.Tests project (xUnit + NSubstitute)
• 29 unit tests covering core domain types and session management

Test coverage

• ResultTests (10) — Success, Failure, Map, Bind, Match, IsSuccess/IsFailure
• ErrorTests (5) — factory methods, ErrorType enum, record equality
• UserSettingsTests (8) — JSON round-trip, defaults, empty/null/invalid input, nested objects
• UserSessionManagerTests (6) — Set/Get/Remove/ContainsKey, cleanup between tests

Closes #12

How to run

dotnet test TelegramMediaRelayBot.Tests/

Test plan

• All 29 tests pass
• dotnet build passes