Add Tests for Manager Component

[KengoWada]

Proposed changes

This PR introduces several improvements and testing utilities for the task management framework:

• Add HandlerRegistry type
  Simplifies and improves clarity for managing task handlers by introducing a dedicated type alias for the handler map.
• Add MockWorker
  Provides a mock implementation of the Worker interface to facilitate unit testing of the Manager. It respects context cancellation and integrates with sync.WaitGroup for shutdown tracking.
• Add accessor methods to Manager
  Adds Workers() and BackoffPolicy() methods to the Manager struct to expose internal components for testing and inspection while maintaining encapsulation.
• Add TestManager
  Introduces a new test suite focused on validating the behavior of Manager, including task publishing, worker lifecycle, and proper interaction with the broker and backoff policy.

Related issue(s)

Fixes #10

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.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.04%. Comparing base (4dd76b0) to head (ddf1247).

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Additional details and impacted files

@@             Coverage Diff              @@
##           develop      #13       +/-   ##
============================================
+ Coverage    41.95%   81.04%   +39.08%     
============================================
  Files            4        4               
  Lines          143      153       +10     
============================================
+ Hits            60      124       +64     
+ Misses          83       29       -54     

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