cmd: refactor SecureServing.Serve return in scheduler and descheduler

[googs1025]

Ⅰ. Describe what this PR does

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

V. Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in make test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.35%. Comparing base (30ea8bb) to head (6b1314a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2551      +/-   ##
==========================================
- Coverage   66.36%   66.35%   -0.02%     
==========================================
  Files         485      485              
  Lines       57098    57098              
==========================================
- Hits        37893    37887       -6     
- Misses      16453    16463      +10     
+ Partials     2752     2748       -4     

Flag	Coverage Δ
unittests	66.35% <ø> (-0.02%)	⬇️

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[ZiMengSheng]

/lgtm
/approve

[ZiMengSheng]

/lgtm
/approve

[koordinator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ZiMengSheng

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