fix(locker): handle card duplication in payouts flow

[kashif-m]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

Explained in #3950

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

Motivation and Context

How did you test it?

Note - DM for postman collection

For ensuring card duplication is handled in payouts flow, we use below procedure (assuming merchant accounts and connector accounts are created)

• Create a new customer
• Save a card for this customer using /payment_methods endpoint
• Create a new payout for this customer using the same card number
• Payouts flow would try to persist these details in locker
• Test # 1 - listing customer's PMs should return a single PM
• Create a new payout using SPM
• Payout flow would try to persist these details again
• Test # 2 - listing customer's PMs should return a single PM
• Try to save the same card details for this user using /payment_methods again
• Test # 3 - listing customer's PMs should return a single PM

Above test cases are present in postman collection.

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible
• I added a CHANGELOG entry if applicable