feat: add optional getters for nullable fields

[jorgerod]

Closes: #24002

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Summary by cubic

Adds an opt‑in flag that makes getters return Optional for non‑required fields in Java and JavaSpring generators. Improves null-safety without changing field or setter types; off by default and does not affect JsonNullable, nullable, or required fields.

• New Features

  • New optionalGettersForNullableFieldsOnly option in AbstractJavaCodegen.
  • Enabled in Java and JavaSpring POJO templates (restclient, resttemplate, webclient): getters return Optional<T>; fields and setters stay raw.
  • Added configs and generated samples for Spring Boot 4 + Jackson 3 + JSpecify (Java clients: restclient, resttemplate) and a Spring server config; docs updated across generator pages; tests added to validate Optional getters.

• Bug Fixes

  • Fixed indentation in generated nullable getters.
  • Propagate discriminator property to subtype models to keep correct annotations and prevent Optional wrapping of discriminators when the option is enabled.

^{Written for commit e32abbb. Summary will update on new commits.}

[cubic-dev-ai]

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