[BUG] [dart-dio] Cannot serialize polymorphic request body

• 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?
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• What's the actual output vs expected output?
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Description

Generated code has a bug when using oneOf for request bodies, you can instantiate the objects fine with their builders, but when passing them into the service for serialization, it throws the following error.

DioException [unknown]: null
Error: type '_Map<String, Object?>' is not a subtype of type 'Iterable<Object?>' in type cast

openapi-generator version

7.0.1

OpenAPI declaration file content or url

I have created a repo specifically to highlight the issue
https://github.com/JakeThomson/openapi-generator-dart-dio-issue

API spec is located here
https://github.com/JakeThomson/openapi-generator-dart-dio-issue/blob/main/api-spec.yaml

Generation Details

I run the following command from the project root
npx @openapitools/openapi-generator-cli generate -i api-spec.yaml -g dart-dio -o ./packages/openapi

Steps to reproduce

1. Clone project linked above
2. Run the test located in /tests/failing_test.dart

Related issues/PRs

Suggest a fix

I haven't been able to work out how to fix it, but on the line below is where the crash happens, we are casting a map to an Iterable.

https://github.com/JakeThomson/openapi-generator-dart-dio-issue/blob/4b6a916799e9b3ec20b099e73d181fd0baa6b466/packages/openapi/lib/src/model/test_request.dart#L102

Start with the linked api-spec.yaml and /tests/failing_test.dart, then inspect the generated packages/openapi/lib/src/model/test_request.dart around line 102. Reproduce the failure with the documented dart-dio generation command and verify that polymorphic request bodies serialize without the reported cast error.