[KOTLIN-SPRING] - add support for 'openApiNullable' - jackson nullable library to handle non-required AND nullable: true properties

[Picazsoo]

Purpose of this PR is to allow proper handling of nullable: true attributes that are not included in the list of required attributes. This is needed to be able to handle e.g. JSON Merge Patch (RFC 7396) - ability to differentiate between omitting an attribute in the json or explicitly setting it to null to either skip modifying it / set it to null.

This is already supported in java spring codegen, so this brings the two spring generators closer to feature parity.
The feature is opt-in by setting 'openApiNullable' to true.

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Summary by cubic

Adds openApiNullable support to Kotlin Spring generators to correctly model optional + nullable properties with a 3‑state type. Enable with openApiNullable: true to use JsonNullable, reject explicit nulls where not allowed, and have the generator add the dependency and module wiring.

• New Features

  • New openApiNullable option for Kotlin Spring generators.
  • Maps required: false, nullable: true to JsonNullable<T> (present/null/absent).
  • Adds @JsonSetter(nulls = Nulls.FAIL) for optional non‑nullable fields.
  • Registers JsonNullableModule bean in the generated Spring Boot app.
  • Adds org.openapitools:jackson-databind-nullable to generated Gradle/Maven files (SB2 uses 0.2.8; SB3/SB4 use 0.2.10).
  • Works with Jackson 2 and Jackson 3 (require jackson-databind-nullable ≥ 0.2.10 for Jackson 3).
  • Docs updated and tests added for the 4 required/nullable states and $ref types; samples regenerated.

• Refactors

  • Centralized OPENAPI_NULLABLE in CodegenConstants and aligned usage across Java/Kotlin codegens.
  • Refined Jackson annotation import mappings for Jackson 3; fixed Kotlin template imports.
  • Improved springBootApplication.mustache structure and conditional module registration.
  • Minor template dependency updates (e.g., Kotlin 1.9.25 in SB3 POMs) and added test dependencies where appropriate.

^{Written for commit 1cbd619. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 270 files

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[cubic-dev-ai]

No issues found across 270 files

_{Note: This PR contains a large number of files. cubic only reviews up to 100 files per PR, so some files may not have been reviewed. cubic prioritizes the most important files to review.
On a pro plan you can use ultrareview for larger PRs.
Re-trigger cubic}

[Picazsoo]

this was not being set anywhere until now as far as I can tell. So on jackson 2 the JsonDeserialize simply was not imported.