[maximeroucher]

Two fixes so custom scalars (e.g. date-time -> DateTime) work end to end, including with separate_models and nullable fields.

• Emits import package:json_annotation/json_annotation.dart as json; in the separate-models header when scalars are configured, so the generated _$XxxJsonConverter classes (which implement json.JsonConverter) compile in the models file.
• Applies the scalar converter annotation to nullable (anyOf) date-time properties (OpenAPI 3.1 anyOf: [date-time, null]), so they serialize through the custom scalar instead of the default DateTime handling.
• Adds a test covering the nullable anyOf date-time property.

Fixes #804. Relates to #840.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.69%. Comparing base (a08d9a4) to head (2eecf0e).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
+ Coverage   63.65%   63.69%   +0.04%     
==========================================
  Files           9        9              
  Lines        1183     1179       -4     
==========================================
- Hits          753      751       -2     
+ Misses        430      428       -2     

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