* Add unit tests for `ReadLine` activity (#7110)
- Implemented `ReadLineTests` to verify the behavior of the `ReadLine` activity.
- Added scenarios for reading input, stream provider usage, and successful completion.
- Refactored `ExecuteAsync` helper to streamline test execution with mock dependencies.
* Enhance `ReadLine` activity tests with additional scenarios: validate null handling, default provider usage, and completion status assertions.
