Refactor merged branch deletion

[gaiksaya]

Description

Refactor merged branch deletion. Works as expected https://github.com/gaiksaya/opensearch-build-libraries/actions/runs/10930031518

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.32%. Comparing base (5cc8d08) to head (9c65083).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #504   +/-   ##
=========================================
  Coverage     84.32%   84.32%           
  Complexity       80       80           
=========================================
  Files           108      108           
  Lines           523      523           
  Branches         61       61           
=========================================
  Hits            441      441           
  Misses           26       26           
  Partials         56       56           

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