Adds support to dart-dio and built value for the new enumUnknownDefaultCase flag. When this flag is enabled, a default enum value is created.

With this change, that value is now set as the fallback value when deserializing unknown enums. This prevents runtime errors when the deserializer encounters an unknown enum.

PR checklist

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

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  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*.
  For Windows users, please run the script in Git BASH.
• [ x] 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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