Fix NoClassDefFoundError for kotlin.reflect on pure-Java projects

[vpelikh]

Problem

A pure-Java Spring Boot project using springdoc-openapi-starter-webmvc-api crashes on GET /v3/api-docs with java.lang.NoClassDefFoundError: kotlin/reflect/full/KClasses.

Root cause: KotlinRequiredPropertyCustomizer was instantiated and registered as a Spring ModelConverter whenever kotlin-stdlib is on the classpath. The outer SpringDocKotlinConfiguration class is gated only by @ConditionalOnClass(Continuation::class) (kotlin-stdlib), but the customizer's resolve() method uses kotlin.reflect.full.* (kotlin-reflect).

In a pure-Java project, kotlin-stdlib is present while kotlin-reflect is absent (both are <scope>provided</scope> in the common module's pom.xml). So the customizer was created, registered as a ModelConverter, and threw NoClassDefFoundError the first time model resolution touched a Pageable/Sort/Page type — exactly the reporter's stack trace (KotlinRequiredPropertyCustomizer.resolve ← PageableOpenAPIConverter.resolve ← PageOpenAPIConverter.resolve ← SortOpenAPIConverter.resolve).

Solution

Moved the kotlinRequiredPropertyCustomizer @Bean from the outer SpringDocKotlinConfiguration into the inner KotlinReflectDependingConfiguration, which is gated by @ConditionalOnClass(name = ["kotlin.reflect.full.KClasses"]). This matches the other three Kotlin customizers already nested there.

Now the bean is only created when kotlin-reflect is present. For pure-Java projects the customizer is never registered as a ModelConverter, so resolve() is never invoked and the NoClassDefFoundError cannot occur.

Result

Pure-Java and other non-Kotlin consumers (kotlin-stdlib present, kotlin-reflect absent) no longer crash: GET /v3/api-docs now resolves Pageable/Sort/Page models and returns the OpenAPI document as expected.

The failure path is eliminated — KotlinRequiredPropertyCustomizer is never registered as a ModelConverter unless kotlin-reflect is on the classpath, so resolve() can no longer throw NoClassDefFoundError: kotlin/reflect/full/KClasses.

Kotlin projects are unaffected: kotlin-reflect is always present for them, so the customizer is still created and the required-property schema behavior is preserved.

Closes #54