Issue:
Bug: [FEATURE]:(doc) Add list mandates for customer in api reference



### Feature Description

Add `list mandates for customer` docs in api reference

### Possible Implementation

Add in open_api spec

### Have you spent some time checking if this feature request has been raised before?

- [X] I checked and didn't find a similar issue

### Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/juspay/hyperswitch/blob/main/docs/CONTRIBUTING.md)

### Are you willing to submit a PR?

Yes, I am willing to submit a PR!

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [X] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
add list mandates for customer

### 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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Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
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Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
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## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
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Can't be tested as this is a doc update PR

## 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:


Can't be tested as this is a doc update PR

Patch:
diff --git a/crates/openapi/src/openapi.rs b/crates/openapi/src/openapi.rs
index 1936300ee14..dc38181a683 100644
--- a/crates/openapi/src/openapi.rs
+++ b/crates/openapi/src/openapi.rs
@@ -114,6 +114,7 @@ Never share your secret api keys. Keep them guarded and secure.
         routes::customers::customers_list,
         routes::customers::customers_update,
         routes::customers::customers_delete,
+        routes::customers::customers_mandates_list,
 
         //Routes for payment methods
         routes::payment_method::create_payment_method_api,
diff --git a/crates/openapi/src/routes/customers.rs b/crates/openapi/src/routes/customers.rs
index 7517cdc61a0..19901cbbeb9 100644
--- a/crates/openapi/src/routes/customers.rs
+++ b/crates/openapi/src/routes/customers.rs
@@ -100,3 +100,19 @@ pub async fn customers_delete() {}
     security(("api_key" = []))
 )]
 pub async fn customers_list() {}
+
+/// Customers - Mandates List
+///
+/// Lists all the mandates for a particular customer id.
+#[utoipa::path(
+    post,
+    path = "/customers/{customer_id}/mandates",
+    responses(
+        (status = 200, description = "List of retrieved mandates for a customer", body = Vec<MandateResponse>),
+        (status = 400, description = "Invalid Data"),
+    ),
+    tag = "Customers Mandates List",
+    operation_id = "List all Mandates for a Customer",
+    security(("api_key" = []))
+)]
+pub async fn customers_mandates_list() {}
diff --git a/openapi/openapi_spec.json b/openapi/openapi_spec.json
index 2803141b38b..be011422e03 100644
--- a/openapi/openapi_spec.json
+++ b/openapi/openapi_spec.json
@@ -1608,6 +1608,39 @@
         ]
       }
     },
+    "/customers/{customer_id}/mandates": {
+      "post": {
+        "tags": [
+          "Customers Mandates List"
+        ],
+        "summary": "Customers - Mandates List",
+        "description": "Customers - Mandates List\n\nLists all the mandates for a particular customer id.",
+        "operationId": "List all Mandates for a Customer",
+        "responses": {
+          "200": {
+            "description": "List of retrieved mandates for a customer",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "array",
+                  "items": {
+                    "$ref": "#/components/schemas/MandateResponse"
+                  }
+                }
+              }
+            }
+          },
+          "400": {
+            "description": "Invalid Data"
+          }
+        },
+        "security": [
+          {
+            "api_key": []
+          }
+        ]
+      }
+    },
     "/customers/{customer_id}/payment_methods": {
       "get": {
         "tags": [