Enables opt-in deduction based polymorphism for oneOf schemas lacking discriminator. Works the same way as in java-spring codegen.

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Summary by cubic

Adds an opt-in setting to generate deduction-based polymorphism for discriminator-free oneOf interfaces in Kotlin Spring using Jackson annotations. Centralizes the option across generators and updates docs.

• New Features
  • Kotlin Spring: for oneOf interfaces without a discriminator, generate @JsonTypeInfo(use = Id.DEDUCTION) and @JsonSubTypes; no change when a discriminator is present.
  • Generators: introduce CodegenConstants.USE_DEDUCTION_FOR_ONE_OF_INTERFACES; wire into SpringCodegen and KotlinSpringServerCodegen.
  • Docs/Tests: document behavior in kotlin-spring, spring, and java-camel; add tests verifying deduction only applies without a discriminator.

^{Written for commit ff5103c. Summary will update on new commits.}