Replace jackson with kotlinx-schema

[michalharakal]

SKaiNET already uses kotlinx.serialization for JSON export in skainet-compile-json.
However, the project still keeps Jackson / JSON Schema validator dependencies in the Gradle version catalog.

We should check whether the remaining Jackson-based schema handling can be replaced with a Kotlin-native solution, preferably [kotlinx-schema](https://github.com/Kotlin/kotlinx-schema), using the existing @Serializable models.

Scope

• Start with the main SKaiNET repository.
• Focus on skainet-compile-json.
• Check the existing JSON export models and tests.
• Add a small proof of concept for generating schema from an existing @Serializable model.
• Keep current JSON export behavior unchanged.
• Note any similar dependency usage in other public SKaiNET repositories, especially SKaiNET-transformers, as follow-up work.

Acceptance criteria

• A Kotlin-native schema generation path exists for at least one existing JSON export model.
• Existing JSON export tests still pass.
• The PR clearly explains whether any Jackson / JSON Schema dependency can be removed now or should remain as follow-up.
• No large refactor or repo-wide migration is required.

Suggested files to check

• gradle/libs.versions.toml
• skainet-compile/skainet-compile-json/build.gradle.kts
• Serialization.kt
• SkJsonModel.kt
• GoldenJsonTest.kt

Notes

kotlinx-schema is still experimental, so the goal is to validate the approach first.
If the migration looks good, we can create follow-up issues for other SKaiNET ecosystem repositories.