cleanup(pubsub): add must_use attribute to publish

[suzmue]

Fixes #4950

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.93%. Comparing base (7b3a369) to head (335936d).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4951      +/-   ##
==========================================
- Coverage   93.94%   93.93%   -0.02%     
==========================================
  Files         221      221              
  Lines        8538     8538              
==========================================
- Hits         8021     8020       -1     
- Misses        517      518       +1     

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[dbolduc]

will clippy let us say:

Suggested change

	let handle = publisher.publish(msg.clone());
	drop(handle);
	let _ = publisher.publish(msg.clone());