[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.
3. Clear registrations and subscriptions. The destructors will free the mDNS library's client-side objects.
4. Trigger state callbacks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.60%. Comparing base (2b41187) to head (af1a692).
Report is 954 commits behind head on main.

Additional details and impacted files

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##             main    #2725       +/-   ##
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- Coverage   55.77%   43.60%   -12.17%     
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  Files          87      108       +21     
  Lines        6890    13355     +6465     
  Branches        0      953      +953     
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+ Hits         3843     5824     +1981     
- Misses       3047     7223     +4176     
- Partials        0      308      +308     

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[abtink]

A general question about how the Stop() method is intended to be used:

1. Stop() can be called when the underlying mDNS provider (e.g., mDNSResponder) is itself stopped.
2. It may be called when the underlying mDNS provider is still active and running, but for some other reason, we are stopping all mDNS operations.

I noticed we have StopMode input with Stop(kNormalStop) or kStopOnServiceNotRunningError, which makes it seem that both cases are used and should be supported.

If we need to support both cases (which I think we need to), then I think we should handle them differently.

• In the first case, it is okay/desired to set mState early to ensure the Clear() methods (destructors and Unregister() calls) won't call mDNSResponder APIs anymore, as it is no longer running.
• In the second case, we do want to call mDNSResponder APIs to clear any previously registered entries (otherwise, it continues to publish the service/host info). But then setting mState early in the method will cause these to be skipped (which would be incorrect).

My suggestion is that we track whether the mDNSResponder is active in a separate variable other than mState, which we then check (along with mState) to decide if we can call any of the mDNSResponder (or Avahi) APIs. Thoughts?