fix(ai-code-review): Add on_new_commit flag

[suejung-sentry]

Use new feature flag to gate rollout of on_new_commit=True so we run code review on every commit (instead of just when PR opened). We'll roll this out to batches of orgs at a time

PREVENT-472

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #104489      +/-   ##
===========================================
- Coverage   80.53%    80.53%   -0.01%     
===========================================
  Files        9350      9350              
  Lines      400099    400108       +9     
  Branches    25660     25660              
===========================================
+ Hits       322216    322221       +5     
- Misses      77415     77419       +4     
  Partials      468       468