#25493 Fix nullable items in nested Kotlin arrays

Summary

• Preserve nullability when Kotlin client models contain nested arrays whose items reference nullable schemas.
• Resolve referenced item schemas when determining nullability while retaining support for inline nullable item schemas.
• Add a regression test and OpenAPI fixture for issue #25471.

Root cause

The Kotlin generator checked nullability on the $ref item schema itself rather than the schema it references. As a result, nullable model items inside nested arrays were generated as non-nullable Kotlin types.

Testing

• ./mvnw -pl modules/openapi-generator -Dtest=KotlinClientCodegenModelTest test

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Summary by cubic

Fixes Kotlin client generation so nullable model items inside nested arrays keep their nullable type instead of being generated as non-nullable.

• Resolves referenced item schemas when checking nullability, while preserving support for inline nullable item schemas.
• Adds a regression test and OpenAPI fixture for issue #25471.