[BUG][JAVA][Spring] 23681 - Add JSpecify @Nullable to fluent setter parameter for optional properties

[Quin-Proctor-GGL]

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Summary by cubic

Add JSpecify @Nullable to fluent setter parameters for optional properties in the JavaSpring generator. This fixes missing nullability on setter parameters and aligns with generated field annotations.

• Bug Fixes
  • In JavaSpring/pojo.mustache, changed setter param from {{{datatypeWithEnum}}} to {{>nullableDataType}} while keeping {{>nullableAnnotation}}, so optional and container types carry correct JSpecify nullability.

^{Written for commit ac01da3. Summary will update on new commits.}