Validate curation draft before update

[dglemos]

Description

Update the endpoint curation/<str:stable_id>/update/ to call method validate_to_save() to check if JSON object is already stored in the curation table for the same user.

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JIRA Ticket

G2P-691

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Contributor Checklist

• The code compiles and runs as expected
• Relevant unit tests are added or updated
• All unit tests are passing
• Code follows the G2P Coding Guidelines
• Documentation (code comments, confluence, etc.) has been updated as needed

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Reviewer Checklist

Please follow the Code Review Guidelines when reviewing this PR.

• I have followed all review guidelines

[seeta-ramaraju]

LGTM