Add tests for dart-dio anyof

[wing328]

a follow up pr to #24605

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Adds a new dart-dio sample and CI coverage for AnyOf (primitive) to verify codegen, serialization, and build stability for the Dart client. Updates samples and workflow to catch regressions in AnyOf handling.

• New Features
  • Added bin/configs/dart-dio-anyof.yaml to generate from built_value_anyof_primitive.yaml.
  • Added sample at samples/openapi3/client/petstore/dart-dio/anyof with generated FieldValue (AnyOf<String|int>), DefaultApi, serializers/docs, and placeholder tests; pubspec.yaml includes dio, one_of, one_of_serializer, and built_value.
  • Updated .github/workflows/samples-dart-build-test.yaml to build/test the new sample.

^{Written for commit 78f1ab1. Summary will update on new commits.}