fix: [JAVA][SPRING] Nullaways warn with JSpecify => add missing annotation to parameter of method toIndentedString

[pkernevez]

PR checklist

fixes #22598

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As suggested for Java Spring:
@cachescrubber (2022/02) @welshm (2022/02) @MelleD (2022/02) @atextor (2022/02) @manedev79 (2022/02) @javisst (2022/02) @borsch (2022/02) @banlevente (2022/02) @Zomzog (2022/09) @martin-mfg (2023/08)

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Summary by cubic

Annotates the toIndentedString(Object) parameter in the Java Spring model template with a nullable annotation to satisfy NullAway/JSpecify and remove warnings. Fixes #22598; updates the test to assert the annotation with no runtime changes.

^{Written for commit b53565d. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

[cubic-dev-ai]

No issues found across 1198 files

Note: This PR contains a large number of files. cubic only reviews up to 75 files per PR, so some files may not have been reviewed.

[wing328]

merged via #22685 with resolved merge conflicts