Add support for `/graphql/realtime` url for Mock

### Is this related to a new or existing framework?

New framework

### Is this related to a new or existing API?

GraphQL API

### Is this related to another service?

_No response_

### Describe the feature you'd like to request

Amplify JS has special handling for Websocket subscription. when the AppSync API is using custom domain. Please add support for the same when using mock. When running mock server a special variable `aws_appsync_dangerously_connect_to_http_endpoint_for_testing` is set to allow to connect using unsecure websocket.

https://github.com/aws-amplify/amplify-js/blob/bb60060d30c7463d1893b7678ebc18e959b46990/packages/pubsub/src/Providers/AWSAppSyncRealTimeProvider.ts#L144-L148

https://github.com/aws-amplify/amplify-js/blob/bb60060d30c7463d1893b7678ebc18e959b46990/packages/pubsub/src/Providers/AWSAppSyncRealTimeProvider.ts#L170-L172

CLI has a bug report(aws-amplify/amplify-category-api#122) to add support to `/graphql/realtime` endpoint and we would like feature parity in the libraries as well

### Describe the solution you'd like

When `aws_appsync_dangerously_connect_to_http_endpoint_for_testing` is set, add `/realtime` to the websocket url

### Describe alternatives you've considered

Keeping the mock subscription server in `/grpahql` but it would degrade the DX

### Additional context

_No response_

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request
- [ ] ⚠️ This feature might incur a breaking change

Read the referenced AWSAppSyncRealTimeProvider.ts sections at lines 144-148 and 170-172 to understand the expected custom-domain behavior. Then locate the mock server's WebSocket URL construction and related subscription tests. Done means mock subscriptions use the requested /graphql/realtime endpoint when the testing connection variable is set, while existing behavior remains unchanged.

Written by the indexing model from the issue text.