fix(gosdk): reject empty JSON responses

[daveshanley]

A generated method with a declared JSON success response currently accepts an empty body and returns a zero-value model. Reject the missing payload as a ResponseDecodeError, preserving status, headers, and body for diagnosis.

Validation: go test ./generator/sdk/gosdk && go vet ./generator/sdk/gosdk

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.87%. Comparing base (64654b7) to head (888c4d9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #632   +/-   ##
=======================================
  Coverage   99.87%   99.87%           
=======================================
  Files         296      296           
  Lines       37147    37175   +28     
=======================================
+ Hits        37101    37129   +28     
  Misses         27       27           
  Partials       19       19           

Flag	Coverage Δ
unittests	99.87% <100.00%> (+<0.01%)	⬆️

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