[dart-dio] fix the issue of multi layer issues with dart-dio code generation

[fstotz]

This fixes the issues with multi layer class using oneOf in dart-dio
fixes #15467

The PR includes test for dart-dio, which are using the example from the ticket

@jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

PR checklist

• Read the contribution guidelines.
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• 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.
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---

Summary by cubic

Fix multi-level oneOf discriminator handling in dart-dio so generated built_value serializers/discriminators work correctly across deep hierarchies. Addresses #15467.

• Bug Fixes
  • Use schema-local discriminator mappings when present; for inherited discriminators keep only true allOf descendants (no sibling leaks).
  • Order mapped models by inheritance depth (subclasses before parents) to pick the most specific type first.
  • Add discriminator vendor extensions (x-discriminator-mapped-models-nonself, x-has-discriminator-self-mapping, x-discriminator-self-mapping-name) and update templates to handle self vs non-self mappings reliably.
  • Add tests with issue_15467.json covering mapping filtering and ordering, and regenerate affected dart-dio samples.

^{Written for commit d9a2139. Summary will update on new commits.}

[cubic-dev-ai]

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