[REQ] [Dart] Add tests for recent PRs

[nickmeinhold]

Is your feature request related to a problem? Please describe.

Several PRs have come through that were fixing small specific issues that would make for excellent tests to add to the CI, some PRs even provided repro steps with an OpenAPI declaration file. An example is #5355.

Describe the solution you'd like

• Go back through recent PRs and determine good candidates for tests
• Create a new OpenAPI declaration file and test runner (follow the existing example that uses the Petstore declaration file)
• add the new tests to CI