fix: Ensure Unsubscribe waits for operation completion using getTokenError

[FelixTing]

fix: #420

PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?)
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?)

Testing Instructions

Run core-command with the go-mod-messaging module from this branch, and verify that sending command requests via message bus works properly.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 57.85%. Comparing base (fedb49d) to head (cc91409).

Files with missing lines	Patch %	Lines
internal/pkg/mqtt/client.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #421      +/-   ##
==========================================
+ Coverage   57.78%   57.85%   +0.07%     
==========================================
  Files          22       22              
  Lines        1163     1165       +2     
==========================================
+ Hits          672      674       +2     
  Misses        437      437              
  Partials       54       54              

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[judehung]

LGTM