client: expose callback function

[rleungx]

What problem does this PR solve?

Issue Number: ref #8690

What is changed and how does it work?

mockPDServiceDiscovery needs it.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JmPotato, lhy1024

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• OWNERS [JmPotato,lhy1024]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-05 10:03:15.554741733 +0000 UTC m=+266067.087153729: ☑️ agreed by lhy1024.
• 2025-02-05 10:17:26.703456078 +0000 UTC m=+266918.235868076: ☑️ agreed by JmPotato.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.21%. Comparing base (2a1f2dc) to head (80ea931).
Report is 2 commits behind head on master.

❌ Your patch status has failed because the patch coverage (66.66%) is below the target coverage (74.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9032      +/-   ##
==========================================
- Coverage   76.26%   76.21%   -0.06%     
==========================================
  Files         466      467       +1     
  Lines       70750    71101     +351     
==========================================
+ Hits        53961    54188     +227     
- Misses      13425    13526     +101     
- Partials     3364     3387      +23     

Flag	Coverage Δ
unittests	76.21% <66.66%> (-0.06%)	⬇️

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