feat(kotlin-client): add Jackson 3 support with useJackson3 option

[yonatankarp]

Summary

• Wire the existing AbstractKotlinCodegen Jackson 3 infrastructure into the kotlin client generator
• When useJackson3=true (requires serializationLibrary=jackson), all templates use the tools.jackson package instead of com.fasterxml.jackson, and build.gradle pulls jackson-module-kotlin 3.0.1 without the separate JSR-310 module
• Register useJackson3 CLI option with validation (requires jackson serialization, incompatible with openApiNullable)
• Replace hardcoded com.fasterxml.jackson with {{jacksonPackage}} in all model, serializer, and library-specific templates
• Make JavaTimeModule conditional (not needed in Jackson 3) for jvm-ktor
• Add tests for validation and generated output
• Add kotlin-jackson3 sample config, generated sample, CI workflow entry, and regenerated docs

Test plan

• Unit tests pass: validation rejects useJackson3 with non-jackson serialization and with openApiNullable
• Generated model files contain import tools.jackson.annotation.JsonProperty (not com.fasterxml.jackson)
• Generated build.gradle contains tools.jackson.module:jackson-module-kotlin and no jackson-datatype-jsr310
• Existing Jackson 2 samples regenerate identically (no diffs)
• CI passes on samples-kotlin-client workflow

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Summary by cubic

Adds Jackson 3 support to the Kotlin client via a new useJackson3 option. Switches templates to tools.jackson for databind/core/module, keeps annotations at com.fasterxml.jackson.annotation, updates Gradle deps, and adjusts the serializer for Jackson 3 API changes.

• New Features

  • Added useJackson3 CLI option with validation (requires serializationLibrary=jackson; incompatible with openApiNullable).
  • Templates use {{jacksonPackage}} across models and JVM clients; annotations stay com.fasterxml.jackson.annotation; jvm-ktor registers JavaTimeModule only for Jackson 2.
  • Serializer template updated for Jackson 3: uses DateTimeFeature and EnumFeature, and removes findAndRegisterModules; SerializationFeature import gated to Jackson 2.
  • Gradle pulls tools.jackson.module:jackson-module-kotlin:3.0.1 when enabled and omits jackson-datatype-jsr310.
  • Added kotlin-jackson3 sample and CI entry; updated generator docs.
  • Tests cover option validation and generated imports/dependencies; Jackson 2 samples regenerate unchanged.

• Migration

  • Enable with: -p serializationLibrary=jackson -p useJackson3=true.
  • Do not combine with openApiNullable.
  • Generated code uses tools.jackson.* for databind/core/module and com.fasterxml.jackson.annotation for annotations; jvm-ktor does not register JavaTimeModule under Jackson 3.

^{Written for commit e657ce3. Summary will update on new commits.}