fix: replace full_clean with clean in ServiceAccountToken save method

[rohan-chaturvedi]

🔍 Overview

Fixed the ApolloError: {'expires_at': ['This field cannot be blank.']} when creating Service Account Tokens that do not expire.

💡 Proposed Changes

Replace the unnecessary full_clean() call with just clean() on model save() to run our custom validation logic

💚 Did You...

• Ensure linting passes (code style checks)?
  - [ ] Update dependencies and lockfiles (if required)
  - [ ] Update migrations (if required)
  - [ ] Regenerate graphql schema and types (if required)
• Verify the app builds locally?
• Manually test the changes on different browsers/devices?

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Note

Replace full_clean() with clean() in ServiceAccountToken.save to run custom validation only.

• Backend – Models (backend/api/models.py):
  • Update ServiceAccountToken.save to call clean() instead of full_clean().

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[Copilot]

Pull Request Overview

This PR fixes an ApolloError when creating Service Account Tokens that do not expire by replacing full_clean() with clean() in the ServiceAccountToken model's save method.

• Replaced full_clean() with clean() in the save method to avoid unnecessary field validation
• Prevents the error "This field cannot be blank" for expires_at when creating non-expiring tokens

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[nimish-ks]

@cursor review