Issue: [ENHANCE] create_user_authentication_method API response to include created entity details

Currently, the [create_user_authentication_method](https://github.com/juspay/hyperswitch/blob/cf30da2df05aac69fd82b676e639b2fbc85dcae5/crates/router/src/core/user.rs#L2482) API endpoint (`POST /user/auth`) returns a generic `200 OK` status with an empty body upon successful creation. This is not informative for API clients, who would benefit from receiving the complete details of the newly created resource in the response, rather than just a success status.

### __Proposed Solution:__

To improve the API's usability and provide immediate access to the created resource's data, the endpoint should return a JSON response containing all the relevant fields of the new user authentication method. This aligns with common REST API practices where a `POST` request to a collection resource returns the representation of the newly created resource.

### __Implementation Plan:__

1. __Define a New Comprehensive Response Struct:__

   - In the file `crates/api_models/src/user.rs`, define a new public struct named `CreateUserAuthenticationMethodResponse`.

   - This struct should derive `Debug`, `serde::Serialize` and any other traits if required.

   - It should contain the following public fields with their respective types:

     - `id`, `auth_id`, `owner_id`,`owner_type`, `auth_type`, `email_domain`,`allow_signup`

   - Place this new struct definition logically near the existing `CreateUserAuthenticationMethodRequest` struct.

2. __Update the Core Function:__

   - In the file `crates/router/src/core/user.rs`, locate the `create_user_authentication_method` async function.
   - Update the function's return type from `UserResponse<()>` to `UserResponse<CreateUserAuthenticationMethodResponse>`.
   - Add the necessary import statement at the top of the file: `use api_models::user::CreateUserAuthenticationMethodResponse;`.
   - Modify the function's success return statement. Instead of `Ok(ApplicationResponse::StatusOk)`, it should return `CreateUserAuthenticationMethodResponse`. This uses the variables already available or computed within the function to populate the response struct.

### __How to Test:__

1. Run the Hyperswitch application locally.
2. Make a `POST` request to the `/user/authentication_method` endpoint.
3. Use the admin API key for authorization. For local development, the key is typically `test_admin`.
4. The request body should be a valid `CreateUserAuthenticationMethodRequest` JSON object.
5. Verify that the response status is `200 OK` and the response body is a JSON object containing the `id`, `auth_id`, `owner_id`, `owner_type`, `auth_type`, `email_domain`, and `allow_signup` of the newly created authentication method.

Curl
```
curl --location '<hyperswitch-backend-base-url>/user/auth' \ 
--header 'Content-Type: application/json' \ 
--header 'api-key: <admin-api-key>' \ 
--data ' 
{ 
"owner_type": "organization",  
"owner_id": "test_123",  
    "auth_method": { 
        "auth_type": "open_id_connect", 
        "private_config": { 
            "base_url": "hello.com", 
            "client_id": "something_client", 
            "client_secret": "something_client", 
            "private_key": "something" 
        }, 
        "public_config": { 
            "name": "okta" 
         
    } 
    }, 
    "allow_signup": true,  
    "email_domain": "None" 
}'
```


### __Expected Outcome:__

After these changes, a successful request to `POST /user/auth` will yield a `200 OK` response with a JSON body similar to the following:

```json
{
  "id": "5500",
  "auth_id": "a1b2c3d4-0-1234-567890abcdef",
  "owner_id": "merchant_123",
  "owner_type": "Merchant",
  "auth_type": "OpenIdConnect",
  "email_domain": "example.com",
  "allow_signup": true
}
```

This provides a much more robust and developer-friendly API experience by giving clients full visibility into the created resource immediately.


Resolution: <think>
PR Context:
- Title: feat: include response body for create_user_authentication_method
- Merged: 2025-10-15T16:19:48Z
- Files changed: 3 files
- Commits: 12 commits

Implementation approach:
#9643 
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [x] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
Currently the API Call to the /user/auth endpoint displays only a 200::OK response code without providing a proper response body containing the authentication details for proper debugging.This PR Introduces a Response body of type CreateUserAuthenticationMethodReponse which co
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat: include response body for create_user_authentication_method (#9653)
Files to modify:


**crates/api_models/src/events/user.rs**
  Modify existing code

**crates/api_models/src/user.rs**
  Add:
    - struct: pub::CreateUserAuthenticationMethodResponse

**crates/router/src/core/user.rs**
  Modify existing code