Add tests for Worker

[KengoWada]

Proposed changes

This pull request adds unit tests for the Worker struct. The tests cover key aspects of the worker's functionality, including task handling, retry logic, and graceful shutdown. The aim is to ensure the worker behaves correctly under different conditions and edge cases.

Changes include:

• Added tests for task handling with valid and invalid tasks.
• Implemented tests to verify retry logic using the backoff policy.
• Created tests to ensure proper shutdown when the context is canceled or the task channel is closed.
• Mocked broker used in the tests to simulate different broker behaviors, such as task consumption and error scenarios.

Related issue(s)

Fixes #4

Checklist

Please ensure that the following checks have been performed:

• I have read and followed the project's contributing guidelines.
• My code follows the project's coding style.
• I have added or updated relevant documentation.
• I have added or updated tests (if applicable).
• All new and existing tests pass.
• My changes do not introduce any new warnings or errors.

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