fix: use mocking MessageClient in unit tests

[judehung]

As real implementation of NewMessageClient will require existing MQTT/NATS broker to access to, the unit tests should use Mocking MessageClient to avoid errors.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 27.59%. Comparing base (94f98a2) to head (65742a1).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4999      +/-   ##
==========================================
- Coverage   27.60%   27.59%   -0.01%     
==========================================
  Files         245      245              
  Lines       17894    17894              
==========================================
- Hits         4939     4938       -1     
  Misses      11828    11828              
- Partials     1127     1128       +1     

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[cloudxxx8]

LGTM