[Spring] fix missing @Nullable on toIndentedString param when useJspecify=true

[SubhamAshok]

With useJspecify=true, toIndentedString(Object o) is generated without @nullable. The nullableAnnotation partial strips the annotation and nothing puts it back, since Object is not wrapped in the jSpecifyDatatype lambda. Without jspecify the output is @nullable Object o, so the two modes disagree.

This matters because the model package is @NullMarked and toString() passes nullable fields into this method. NullAway and IntelliJ report an error on every generated model.

Fix: wrap Object in the jSpecifyDatatype lambda, same pattern as the rest of the template. This complements #24563 which fixes the constructor and fluent setter spots. Relates to #23848.

Checked both modes. jspecify=true now gives @nullable Object o with the import. jspecify=false output is byte identical to before. Added an assertion to SpringCodegenTest#testJspecify, all 8 combos pass. Regenerated samples for bin/configs/spring-boot-4-jspecify.yaml, the diff is one line.

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  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (Scoped sample regeneration to the affected config, spring-boot-4-jspecify.yaml. No generator options or docs changed, so export_docs_generators was not needed.)
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Summary by cubic

Fixes missing @nullable on toIndentedString(Object o) when useJspecify=true by hardcoding the annotation in the Spring POJO template. This aligns jspecify and non-jspecify outputs and removes nullness errors in @NullMarked models. Complements #24563 and relates to #23848.

• Bug Fixes
  • Hardcode @Nullable on toIndentedString(@Nullable Object o) in pojo.mustache instead of using the nullableAnnotation partial, avoiding context side effects and applying the fix to use-optional variants too.
  • Add a test assertion for the method signature and the import of org.jspecify.annotations.Nullable; regenerate affected samples.

^{Written for commit 6bd8ee2. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[jpfinne]

@SubhamAshok I think everyone makes it too complex by using {{>nullableAnnotation}} For example #22685 and #22599
The partial is intended to be used for a CodeGenProperty.

It gives side effects with the complex conditions on the property (required, isNullable, defaultValue....)
Let keep it simple: String toIndentedString(@Nullable Object o)

My PR is work in progress: #24563

[SubhamAshok]

Thanks, makes sense. #22599 / #22685 show how the partial got here and why it keeps breaking. Updated to a hardcoded @Nullable Object o. Note: useOptional configs previously got no annotation here, now they do, so a few more sample files changed. All spring samples regenerated.

[jpfinne]

@SubhamAshok My PR is ready #24563

It includes the same fix for toIndentedString, and a lot more for fluent setters...

Can you review?