REST: Add missing text domain and fix typos in REST API controller strings #643

Description

In the REST API controllers (includes/restapi/):

• None of the __( '...' ) translation calls pass the plugin's text domain ('wp-feature-notifications'). This prevents those strings from being properly localized according to WordPress i18n standards.
• Several error messages and schema descriptions contain grammatical errors, missing words, and extra punctuation:
  • includes/restapi/class-channel-controller.php:
    • __( 'Sorry, you must be logged to view channels.' ) → missing "in".
    • __( 'The default view context the notification channel.' ) → missing "of".
  • includes/restapi/class-notification-controller.php:
    • __( 'Sorry, you must be logged check notifications.' ) → missing "in to".
    • __( 'The URL of the accept action..' ) → trailing double period.
    • __( 'The view context the notification.' ) → missing "of".
    • __( 'The identifier of user the notification is belongs to.' ) → missing "the" and erroneous "is".
  • includes/restapi/class-subscription-controller.php:
    • __( 'The identifier of user the subscription is belongs to.' ) → missing "the" and erroneous "is".

Step-by-step reproduction instructions

1. Inspect includes/restapi/class-channel-controller.php, class-notification-controller.php, and class-subscription-controller.php.
2. Notice all __( ... ) calls omit the text domain argument.

Proposed solution

Add 'wp-feature-notifications' text domain to all translation calls in includes/restapi/ and fix the grammatical issues in the strings.