Feature/fix issues

[Picazsoo]

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.

---

Summary by cubic

Fixes compile-breaking Kotlin output for allOf/discriminator inheritance hierarchies in the kotlin-spring generator, and adds an opt-in fixPolymorphicInheritance flag that promotes non-discriminated allOf parents to interface so subtypes can legally extend them. Also fixes three pre-existing, always-on compile regressions found in real-world specs.

Bug Fixes

• Subtypes re-declaring a property inherited through an allOf-composed parent now get the required override modifier.
• Discriminator properties narrowed to a per-subtype enum are normalized back to the parent's String type for allOf children.
• Free-form/map-typed schemas with a discriminator now render as open class instead of interface : HashMap<...>(), which Kotlin forbids.
• Self-mapped discriminator roots get a synthetic concrete <Schema>Impl leaf so Jackson can construct them.
• anyOf single-refs to a discriminated schema no longer emit override without a real Kotlin parent.
• oneOf/discriminator grouping interfaces with no own properties now synthesize a computed override for their abstract discriminator property.
• Inherited enum properties are typed via the model that actually declares the nested enum, not the composing child.
• fixPolymorphicInheritance interfaces used in deduction-based oneOf unions now redirect @JsonSubTypes to the synthetic Impl leaf.

New Features

• fixPolymorphicInheritance (default false) renders a non-discriminated allOf parent as an interface instead of a final data class, so affected schemas can no longer be instantiated directly.
• Interfaces used as discriminator mapping values get a synthetic <Schema>Impl data class in its own file, named collision-free (Impl, Impl2, ...), suppressible via schemaMapping.

^{Written for commit 5c8412a. Summary will update on new commits.}