fix(pubsub): increase default publish batch limit to 1 MB

[suzmue]

Update the defaults to match the canonical default batch limit for Pub/Sub client libraries.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (d94f006) to head (492e14f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4802   +/-   ##
=======================================
  Coverage   95.01%   95.01%           
=======================================
  Files         204      204           
  Lines        7877     7877           
=======================================
  Hits         7484     7484           
  Misses        393      393           

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