Missing config schema

Problem/Motivation

The module ships default configuration (config/install/user_update_notify.settings.yml) but has no config schema. This causes two problems:

• Kernel and Functional tests check all installed config against its schema by default ($strictConfigSchema = TRUE). Any test that installs the module fails with a SchemaIncompleteException ("No schema for user_update_notify.settings"). Contrib or custom projects that enable this module in their own tests hit the same failure.
• Without a schema, config values are not cast to the right types, cannot be translated (e.g. the email subject and body), and are not validated.

Also, enable is stored as an integer (0) in the default config, and the settings form saves the raw checkbox value instead of a boolean.

Steps to reproduce

1. Create a minimal Kernel test that has protected static $modules = ['user', 'system', 'token', 'user_update_notify']; and calls $this->installConfig(['user_update_notify']);.
2. Run the test.
3. The test fails with Drupal\Core\Config\Schema\SchemaIncompleteException: No schema for user_update_notify.settings.

Alternatively, install Config Inspector and look at user_update_notify.settings: it is reported as having no schema.

Proposed resolution

• Add config/schema/user_update_notify.schema.yml describing user_update_notify.settings:
  • enable: boolean
  • admin_email_recipient, recipient_role, targeting_type: string
  • recipient_email: email
  • email_subject: label (translatable)
  • email_body: text (translatable)
  • target_roles, selected_fields: sequence of string
• Change enable: 0 to enable: false in the default config.
• Cast the checkbox value to (bool) when saving enable in SettingsForm::submitForm().

Remaining tasks

• Review the MR.
• Decide whether an update hook should convert enable on existing sites now. Otherwise the value is cast to a boolean the next time the settings form is saved.
• Add test coverage (follow-up). With the schema in place, Kernel/Functional tests that install the module become possible.

User interface changes

None.

API changes

None.

Data model changes

A config schema is added for user_update_notify.settings. enable is now stored as a boolean instead of an integer. The keys and structure of the config are unchanged.