Write Unit Tests and Integration Tests [93-8]

[mehalyna]

• Task: Write unit and integration tests for OAuth login and JWT token issuance.
• Steps:
  1. Test login flow via Google/GitHub.
  2. Verify that the JWT token is issued upon successful OAuth authentication.
  3. Test user creation and updates during OAuth login.
• Deliverable: Tests written and passing successfully.

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US #93