Issue:
Bug: [BUG] unmask last4 digits of card when listing payment methods for customer



When metadata of a saved card is updated and customer payment methods are listed, the updated card's last 4 digit is getting masked in the response. Instead unmask it and return the raw digits.

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
When metadata of a saved card is updated and customer payment methods are listed, the updated card's last 4 digit is getting masked in the response. This PR unmasks it and returns the raw digits.

Bug: `last4_digits` is masked -

![image](https://github.com/juspay/hyperswitch/assets/70657455/6c3b9af6-a584-4c76-9645-aae71e004a4b)

Fix: unmasked - 

![image](https://github.com/juspay/hyperswitch/assets/70657455/bc5712f5-bd7f-463c-abb4-5f78a5505215)


### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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## Motivation and Context
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## How did you test it?
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1. Save a card in locker
2. Change the metadata of the saved card and try to save it again in locker
3. Now when `list_customer_payment_method` is called, the `last4_digits` field of the updated card has to be unmasked

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable


Testing:

1. Save a card in locker
2. Change the metadata of the saved card and try to save it again in locker
3. Now when `list_customer_payment_method` is called, the `last4_digits` field of the updated card has to be unmasked

Patch:
diff --git a/crates/router/src/core/payment_methods/cards.rs b/crates/router/src/core/payment_methods/cards.rs
index 152232d2dec..4f6d6b4c62b 100644
--- a/crates/router/src/core/payment_methods/cards.rs
+++ b/crates/router/src/core/payment_methods/cards.rs
@@ -248,11 +248,7 @@ pub async fn add_payment_method(
                         Ok(pm) => {
                             let updated_card = Some(api::CardDetailFromLocker {
                                 scheme: None,
-                                last4_digits: Some(
-                                    card.card_number
-                                        .to_string()
-                                        .split_off(card.card_number.to_string().len() - 4),
-                                ),
+                                last4_digits: Some(card.card_number.clone().get_last4()),
                                 issuer_country: None,
                                 card_number: Some(card.card_number),
                                 expiry_month: Some(card.card_exp_month),