[BUG] [JAVA] [retrofit2] No serializationLibrary supplied → defunct generated code?

[Philzen]

Discovered this while working on #18811.

Basically #16853 added the necessary checks for {{#gson}} or {{#jackson}} to the templates, but the generator code never set a proper default, so this bug was first released with 7.4.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Suggest a fix

The RETROFIT_2 execution path in JavaClientCodegen needs to additionalProperties.put(SERIALIZATION_LIB_XY, "true").

[Philzen]

This was a faulty assumption. Adding a test so one doesn't have to speculate in the future: d051d6f