refactor(pubsub): use enum for exactly-once message status

[suzmue]

Replaces the pending and nacking booleans in ExactlyOnceInfo with a MessageStatus enum. This enforces mutual exclusivity of the states and makes the code more idiomatic.

The detailed comments explaining the lease extension behavior for each state have been preserved on the enum variants. Tests have been updated to use the new enum.

For #5252

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.78%. Comparing base (8fc99b3) to head (49afd75).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5338      +/-   ##
==========================================
+ Coverage   97.77%   97.78%   +0.01%     
==========================================
  Files         222      222              
  Lines       46113    46150      +37     
==========================================
+ Hits        45089    45130      +41     
+ Misses       1024     1020       -4     

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[dbolduc]

Good idea