[dart][dart-dio] Fix missing byte response cast

There doesn't seem to be an example for this. Fixes https://github.com/OpenAPITools/openapi-generator/issues/8878

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• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

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CC @agilob