[Mattias-Sehlstedt]

Moves isolated and distinct discriminator logic from the DefaultCodegen to the dedicated DiscriminatorUtils helper. After this change there are three main discriminator-only methods left, createDiscriminator, getAllOfDescendants and getOneOfAnyOfDescendants.

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.
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[cubic-dev-ai]

All reported issues were addressed across 2 files

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