fix(core-api): fix delegate.lastBlock.timestamp property

[rainydio]

Summary

Follow up to #3931 that fixes delegate.lastBlock.timestamp response property.

Checklist

• Ready to be merged

[codecov]

Codecov Report

Merging #4019 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #4019   +/-   ##
========================================
  Coverage    98.47%   98.47%           
========================================
  Files          638      638           
  Lines        14806    14806           
  Branches      1748     1748           
========================================
  Hits         14580    14580           
  Misses         130      130           
  Partials        96       96           

Flag	Coverage Δ
#functional	6.93% <ø> (ø)
#integration	11.52% <ø> (ø)
#unit	95.79% <ø> (ø)

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Impacted Files	Coverage Δ
...s/core-api/src/services/delegate-search-service.ts	100.00% <ø> (ø)

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