fix: Implement unit tests and a model validator for the PaymentInfo pydantic model

[Sulai-maan]

Description

I implemented a model validator that checks that the payment card has not expired. The payment card model previously only validated the character counts of the card number and cvc fields. Also, I replaced the now deprecated validator decorator with the field_validator decorator. Also,

Related Issue (Link to issue ticket)

#1218

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Motivation and Context

We can now avoid callling the Stripe API with expired payment cards and save resources.

How Has This Been Tested?

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Created and ran unit testing of the model using the pytest module.

Screenshots (if appropriate - Postman, etc):

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
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Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• [x ] I have added tests to cover my changes.
• All new and existing tests passed.

[samuelogboye]

Please explain why this implementation is totally different from the approved issue

[Sulai-maan]

I had spotted the issue while working on my approved issue - which required the PaymentInfo model. In hindsight, I should have created a different issue for it. Will do that now. Apologies.

[Sulai-maan]

@samuelogboye I have updated the linked issue for this PR but I can't seem to be able to reopen. Please take a look.