[JAVA] fix jspecify in java builders and toIndentedString

[jpfinne]

Fix some missing @Nullable annotations for java clients:

• add in builders
• add in toIndentedString()

Add a sample for openapiNullable

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Summary by cubic

Fixes JSpecify nullability in generated Java and Spring Boot 4 code so builder chain setters and all‑args constructors accept @nullable where expected, including for fully qualified types and generics. Adds new openApiNullable/useOptional sample configs, updates workflows/tests/samples, and cleans up generated FILES lists.

• New Features

  • Apply @Nullable in builder setters via new nullableArgument_builder for Java and Spring (respects useOptional and optionalAcceptNullable in Spring).
  • Add @Nullable to Spring all‑args constructors when useJspecify is enabled.
  • Mark toIndentedString(Object o) as @Nullable across Java templates and all library variants.
  • New sample configs: spring-boot-4-jspecify-openapiNullable, spring-boot-4-jspecify-useOptional, and java-restclient-springBoot4-jackson3-jspecify-openapiNullable; CI workflows include them.

• Bug Fixes

  • Correct enum nullable handling by using nullableDatatypeWithEnum in nullableArgumentWithEnum.mustache.
  • Fix JSpecify annotation placement with generics using getLastIndex in AbstractJavaCodegen.
  • Update nullable_var_annotations and Spring nullableAnnotation logic to honor useJspecify, openApiNullable, and useOptional (split into default and JSpecify variants).
  • Exclude tests from generated .openapi-generator/FILES to reduce noise in CI.

^{Written for commit c7cbd25. Summary will update on new commits.}