fix: service account token creation bug

[nimish-ks]

🔍 Overview

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

💡 Proposed Changes

• Updated the expires_at field in the ServiceAccountToken model to allow blank values, enhancing flexibility in token management.
• Added a deprecation comment for legacy app-linked service tokens in the ServiceToken model.

🧪 Testing

• Create a Service Account Token with no expiry - verify that creation is successful and a valid token is minted
• Create a Service Account Token with expiry - verify that creation succeeds and a valid token is minted

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Note

Permits non-expiring service account tokens by allowing blank expires_at and adds a deprecation note to legacy app-linked service tokens.

• Backend/models:
  • ServiceAccountToken: set expires_at to blank=True to support non-expiring tokens.
  • ServiceToken: add deprecation comment indicating legacy app-linked tokens.

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[nimish-ks]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no bugs!

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[rohan-chaturvedi]

This is a bandaid fix that will require a migration, and allow blank values for the expiry that we do want to allow. Made a separate fix in #665