Allow Backend-Only Amplify Deployments Without Frontend Definition

### Before opening, please confirm:

- [x] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [x] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [x] I have removed any sensitive information from my code snippets and submission.

### Amplify Hosting feature

Build settings

### Is your feature request related to a problem? Please describe:

Currently, deploying an Amplify app without a frontend build definition results in the following error:
!!! CustomerError: Missing frontend definition in buildspec
This error occurs even when the intention is to deploy only the backend.

### Describe how you'd like this feature to work

Add a setting in Amplify app config to disable frontend build steps ("frontend": false). Enable support for backend-only deployments without requiring a frontend definition in amplify.yml.

Start by reviewing the Amplify Hosting build-settings handling and how amplify.yml is processed when a frontend definition is missing. Confirm the expected behavior for an app configured with "frontend": false, then verify that backend-only deployments succeed without a frontend definition while normal frontend deployments remain supported.

Written by the indexing model from the issue text.