Bug: [BUG] Prevent unwarranted 404 responses for `CustomerPaymentMethodsList` endpoint



`CustomerPaymentMethodsList` currently lists all the saved payment methods for a customer,

However if the customer doesn't have any saved method we end up returning 404...

We should instead return an empty array when the customer doesn't have any payment methods.. and reserve 404 for when the customer id is invalid

We can update the functionality in [this](https://github.com/juspay/hyperswitch/blob/b002c97c9c11f7d725aa7ab5b29d49988baa6aea/crates/router/src/core/payment_methods/cards.rs#L1558) function to solve this use case
diff --git a/crates/router/src/core/payment_methods/cards.rs b/crates/router/src/core/payment_methods/cards.rs
index 499056aa4e5..a5a5c0917fa 100644
--- a/crates/router/src/core/payment_methods/cards.rs
+++ b/crates/router/src/core/payment_methods/cards.rs
@@ -1562,6 +1562,10 @@ pub async fn list_customer_payment_method(
 ) -> errors::RouterResponse<api::CustomerPaymentMethodsListResponse> {
     let db = &*state.store;
 
+    db.find_customer_by_customer_id_merchant_id(customer_id, &merchant_account.merchant_id)
+        .await
+        .to_not_found_response(errors::ApiErrorResponse::CustomerNotFound)?;
+
     let resp = db
         .find_payment_method_by_customer_id_merchant_id_list(
             customer_id,
@@ -1570,11 +1574,6 @@ pub async fn list_customer_payment_method(
         .await
         .to_not_found_response(errors::ApiErrorResponse::PaymentMethodNotFound)?;
     //let mca = query::find_mca_by_merchant_id(conn, &merchant_account.merchant_id)?;
-    if resp.is_empty() {
-        return Err(error_stack::report!(
-            errors::ApiErrorResponse::PaymentMethodNotFound
-        ));
-    }
     let mut customer_pms = Vec::new();
     for pm in resp.into_iter() {
         let parent_payment_method_token = generate_id(consts::ID_LENGTH, "token");

## 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 -->
This pull request updates the `list_customer_payment_method` function to modify its behavior when a customer doesn't have any saved payment methods. Currently, it returns a 404 error, which is misleading because the error is actually due to the lack of payment methods rather than an invalid customer ID. This update aims to provide a more accurate response by returning an empty array when there are no payment methods associated with the customer.


### Additional Changes

- [x] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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2. `crates/router/src/configs`
3. `loadtest/config`
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## Motivation and Context
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## Checklist
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- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable
