Make segment deletion task idempotent

[linhvo229]

Summary

• Make segments.tasks.delete_segment ignore already-deleted/missing segments by catching Segment.DoesNotExist
• Add focused unit tests for existing segment deletion and missing segment no-op behavior

Fixes #7354.

Testing

• git diff --check origin/main..HEAD
• poetry run pytest tests/unit/segments/test_unit_segments_tasks.py -q
• poetry run ruff format segments/tasks.py tests/unit/segments/test_unit_segments_tasks.py --check
• poetry run ruff check segments/tasks.py tests/unit/segments/test_unit_segments_tasks.py

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (413c9b8) to head (755ba14).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7380      +/-   ##
==========================================
- Coverage   98.44%   98.37%   -0.07%     
==========================================
  Files        1398     1399       +1     
  Lines       52628    52522     -106     
==========================================
- Hits        51808    51667     -141     
- Misses        820      855      +35     

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