feat: enhance test example to use dependency injection, useEffect, and scope.launch()

[mkienenb]

The current example React testing code is too minimal to reflect real-world usage patterns, which makes it harder to validate and demonstrate correct behavior in production-like scenarios.

Enhancing the examples to include:

• Dependency Injection
  Allows components to receive services (API clients, stores, etc.) in a testable way. This avoids hidden globals and makes it possible to substitute mocks/fakes cleanly in tests.

• useEffect usage
  Most non-trivial React components rely on side effects (data loading, subscriptions, initialization). Including useEffect ensures examples cover lifecycle behavior and async updates, which are often the source of bugs.

• Avoiding MainScope usage in tests / using test-controlled coroutine scope
  Using MainScope (or other default, unmanaged scopes) inside components makes tests unreliable because:

  • the lifecycle is not tied to the test, so coroutines may outlive the test or leak
  • tests cannot easily control or advance coroutine execution (for the case of runTest)

Together, these changes make the examples:

• more representative of real applications
• easier to test reliably

This will significantly improve the usefulness of the examples as a reference for users building non-trivial applications.

.... And it will also make it easier to provide a reproducible environment for the coroutines issue, but that's not enough justification by itself.

[turansky]

@mkienenb could you, please, describe the goal?
Is it for coroutine's issue?

[turansky]

Non-strict contracts aren't welcome in examples :(
Discussion required