fix(gen): rewrite multipart form generation

[tdakkota]

Fixes #870

[codecov]

Codecov Report

Patch coverage: 85.71% and project coverage change: +0.04 🎉

Comparison is base (a87baa4) 75.69% compared to head (998d9bd) 75.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #872      +/-   ##
==========================================
+ Coverage   75.69%   75.73%   +0.04%     
==========================================
  Files         189      189              
  Lines       13806    13821      +15     
==========================================
+ Hits        10450    10468      +18     
+ Misses       2828     2826       -2     
+ Partials      528      527       -1     

Impacted Files	Coverage Δ
gen/schema_gen.go	81.02% <25.00%> (-0.62%)	⬇️
gen/tstorage.go	59.80% <57.14%> (+0.39%)	⬆️
gen/gen_schema.go	76.56% <90.00%> (+5.40%)	⬆️
gen/gen_contents.go	73.88% <91.66%> (+0.73%)	⬆️
gen/gen_parameters.go	64.77% <100.00%> (ø)
gen/genctx.go	90.90% <100.00%> (+0.28%)	⬆️

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