feat(dart): update the Dart SDK version

[devoncarew]

• update the package deps to use Dart SDK 3.9
• regenerate using exactly 3.9.2
• rev to the latest version of sidekick (which will generate packages for 3.9)
• regenerate all the Dart packages

Done as two commits (the manual changes and the regeneration changes). The formatting changes in the generated code are due to formatter changing from 3.6 to 3.7 (and a few more from 3.7 to 3.8).

cc @brianquinlan

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (34cf3bc) to head (05064f9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3150   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files         108      108           
  Lines        4407     4407           
=======================================
  Hits         4254     4254           
  Misses        153      153           

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