Disallow multiple Domain Aliases for the same environment

Problem / Motivation

The Domain Alias module currently allows creating multiple aliases for the same environment on a single Domain entity. This can lead to unpredictable behavior, as Drupal arbitrarily selects the first matching alias when loading a domain. This affects both web requests and Drush commands.

Proposed Resolution

• Prevent creation of more than one alias per environment per Domain entity.
• Add form/API validation to enforce this rule.
• Optionally, show an alert or warning in the admin UI when conflicts exist.
• Update documentation to clearly state that only one alias is allowed per environment.

Benefits

• Removes ambiguity in domain resolution.
• Ensures predictable behavior for both web and CLI requests.
• Improves clarity and maintainability of Domain Alias configuration.

Remaining Tasks

• Implement validation for creating/editing Domain Alias entities.
• Provide a way to detect and resolve existing conflicts in the configuration.