[mdns] unify the behavior of Stop() between avahi and mDNSResponder

[superwhd]

In the Stop() method, the Publisher will:

1. Early exit if it's already stopped.
2. Mark the state as stopped and trigger callbacks.
3. Clear registrations and subscriptions. The destructors will free the mDNS library's client-side objects.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 45.21%. Comparing base (2b41187) to head (6e1f5c9).
Report is 952 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mdns/mdns_avahi.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2725       +/-   ##
===========================================
- Coverage   55.77%   45.21%   -10.57%     
===========================================
  Files          87      108       +21     
  Lines        6890    13417     +6527     
  Branches        0      964      +964     
===========================================
+ Hits         3843     6066     +2223     
- Misses       3047     7025     +3978     
- Partials        0      326      +326     

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[bukepo]

should this callback be called after everything is cleared?

[bukepo]

Is it better to clear states and then update the state and trigger callback?