Fix REST API scalar input parameter handling

[Sukhendu2002]

Trac ticket: https://core.trac.wordpress.org/ticket/61061

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[github-actions]

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[github-actions]

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[audrasjb]

Hello @Sukhendu2002 thanks for the PR. It has some conflicts, would you mind rebasing it so it can be tested? Thanks!

[Sukhendu2002]

Hi @audrasjb, I've rebased the PR and resolved the conflicts. It should be ready for testing now. Let me know if you need any further changes. Thanks!