feat(kotlin): support runtime Kotlin JSON type tokens

[chaokunyang]

Framework callbacks cannot use a reified Kotlin type parameter, and wrapping their Java Type loses the Kotlin nullability needed to serialize generic responses such as Rt<List<Employee>>.

Add jsonTypeRef<T>(KType) using the existing structural type conversion, so callbacks can retain the controller's declared Kotlin type and pass it to Fory. The reified helper uses the same entry point. A dedicated integration guide introduces spring-fory and explains Kotlin runtime type discovery and Spring adapter requirements. kotlin-reflect remains an application dependency and is added only to tests here.

Fixes #4053.

Validation:

• All 121 fory-json-kotlin tests passed with Kotlin 2.3.20 / JDK 21, including reflected generic roots, nested nullability, defaults, unsigned values, and incomplete type rejection.
• Spotless and Markdown formatting passed.
• The documented external-consumer example compiled and round-tripped successfully with Kotlin 2.4.10 / JDK 26.
• No complete Spring application was run.

[ayush00git]

LGTM