fix(gen): encode discriminator field only once

[tdakkota]

Fixes #1229

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.87%. Comparing base (fbe6d9e) to head (b1ddda2).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1232      +/-   ##
==========================================
+ Coverage   73.80%   73.87%   +0.06%     
==========================================
  Files         189      189              
  Lines       12762    12772      +10     
==========================================
+ Hits         9419     9435      +16     
+ Misses       2802     2797       -5     
+ Partials      541      540       -1     

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