Reorganize tests in prep for adding more

[DTCurrie]

Before we add more unit tests to the repository, I am reorganizing things to help us have consistent and maintainable tests, as well as some utilities to make writing tests easier. I know this is a large PR, but it keeps all of current tests and just updates them to follow recommended best practices for Vitest/TypeScript projects.

All tests follow the Arrange-Act-Assert pattern with explicit comments (// Arrange, // Act, // Assert) to clearly delineate test phases. We prioritize per-test mock setup using helper functions (e.g., setupDialWebRTCMocks()) over beforeEach() hooks to make tests self-contained. Parameterized tests using it.each() reduce duplication, and all mocks are cleaned up in afterEach() with vi.restoreAllMocks(). Critically, we've eliminated all test logic anti-patterns—no loops, conditionals, or mutable state variables—instead using Vitest's declarative chaining API (.mockReturnValueOnce(), .mockImplementationOnce()) for predictable test behavior.

We've adopted a three-directory structure: __tests__/ contains test files (*.spec.ts), __fixtures__/ contains test data and configuration objects as individual named constants (e.g., withAccessToken, testCredential), and __mocks__/ contains reusable mock factory functions that return fresh instances (e.g., createMockPeerConnection()). The key distinction is that fixtures are data (configurations, constants, sample values) while mocks are behavior (factory functions that create test doubles with methods). We use stubs when we only need canned responses and verify the resulting state, and mocks when we need to verify specific interactions occurred. Fake timers (vi.useFakeTimers()) ensure deterministic testing of time-dependent code. This separation provides clear boundaries while enabling reusability and preventing state pollution between tests. It also keeps domain directories clean and focused on "business logic" and keeps domain-related test logic in clear and consistent locations.