fix: Address AmendmentBlockHandler flakiness in old ETL tests

[godexsoft]

As the comment in the code suggests, this is merely a temporary workaround for a flaky test that will be removed in favour of ETLng.

[Copilot]

Pull Request Overview

Temporary workaround to stabilize a flaky AmendmentBlockHandler test by increasing the io_context runtime; intended to be removed with the old ETL deprecation in favor of ETLng.

• Add explanatory comment about test flakiness and future removal.
• Replace conditional run durations (1ms/10ms) with a fixed 50ms run to reduce flakiness.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (b361e3a) to head (dbc998f).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2694   +/-   ##
========================================
  Coverage    79.56%   79.56%           
========================================
  Files          385      385           
  Lines        16153    16153           
  Branches      8459     8459           
========================================
  Hits         12852    12852           
  Misses        2012     2012           
  Partials      1289     1289           

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