Fix response type always nullable under generateNullableReferenceTypes

[christianhelle]

This pull request fixes the issue where nullable types were being returned from the Refit interface when the setting generateNullableReferenceTypes is enabled.

This also adds tests for generating code with non-nullable return types and includes an assert for nullable return type tests.

Additionally, it updates the code to generate non-nullable return types in the ApiResponse.

This resolves #302

[sonarqubecloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2e6a261) 97.30% compared to head (060d88d) 97.33%.
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #304      +/-   ##
==========================================
+ Coverage   97.30%   97.33%   +0.02%     
==========================================
  Files          63       63              
  Lines        2377     2398      +21     
==========================================
+ Hits         2313     2334      +21     
  Misses         40       40              
  Partials       24       24              

Flag	Coverage Δ
unittests	97.33% <100.00%> (+0.02%)	⬆️

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