feat(pubsub): add nack() method to Handler for AtLeastOnce

[suzmue]

Previously, nacking a message in the Pub/Sub subscriber required dropping the handler (drop(h)). While effective, this is less discoverable and readable than an explicit nack(). This change introduces explicit nack() methods to the message handlers for Handler and AtLeastOnce. There is also a private nack() added to ExactlyOnce. Additionally, this change updates the module and method documentation to use the new nack() methods instead of drop().

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.98%. Comparing base (8cc7ac3) to head (801ff6f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5261      +/-   ##
==========================================
- Coverage   97.98%   97.98%   -0.01%     
==========================================
  Files         215      215              
  Lines       44904    44957      +53     
==========================================
+ Hits        44000    44051      +51     
- Misses        904      906       +2     

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