Update backoff strategy to exponential backoff with full jitter

[KengoWada]

Proposed changes

This pull request proposes the following changes:

• Updated the BackoffPolicy.Calculate method to implement exponential backoff with full jitter, improving retry behavior for failed tasks.
• Revised associated tests to validate the new backoff calculation logic and ensure correctness across different retry counts.
• Updated example code to demonstrate how the revised backoff logic behaves in real usage, reflecting the latest retry mechanism.

Related issue(s)

Fixes #14

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 80.00%. Comparing base (a2a02ec) to head (b27728a).

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #15      +/-   ##
===========================================
- Coverage    80.66%   80.00%   -0.67%     
===========================================
  Files            4        4              
  Lines          150      145       -5     
===========================================
- Hits           121      116       -5     
  Misses          29       29              

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