[C#] chore: Turn State unit tests (#1199)

## Linked issues

closes: #1119  (issue number)

## Details
- Added unit tests for each method in the `TurnState` class.
- Increased dotnet coverage by 1.2%.

## Attestation Checklist

- [x] My code follows the style guidelines of this project

- I have checked for/fixed spelling, linting, and other errors
- I have commented my code for clarity
- I have made corresponding changes to the documentation (updating the
doc strings in the code is sufficient)
- My changes generate no new warnings
- I have added tests that validates my changes, and provides sufficient
test coverage. I have tested with:
  - Local testing
  - E2E testing in Teams
- New and existing unit tests pass locally with my changes