Settings form field values collide with monitoring_mail when both modules are enabled

Problem/Motivation

When both monitoring_mail and monitoring_slack are enabled, the two modules alter the same monitoring_settings form without setting #tree on their fieldsets. Both add a severities checkboxes element at the top level, so Drupal generates identical HTML input names (severities[OK], severities[WARNING], …) for both fieldsets.

Result:

• Only one severities value exists in $form_state — both submit handlers read the same value.
• You cannot have different severities for email and Slack notifications.
• Saving monitoring.settings writes the same severities into both monitoring_mail.settings and monitoring_slack.settings.

Steps to reproduce

1. Enable monitoring, monitoring_mail, monitoring_slack.
2. Go to Configuration → System → Monitoring settings.
3. Tick WARNING under Mail notifications and CRITICAL under Slack notifications.
4. Save.
5. Reload — both fieldsets show the same severities.

Proposed resolution

Add #tree => TRUE on the slack fieldset so all Slack fields namespace under slack[*]. Update the submit handler, the test-button submit handler and #limit_validation_errors to use the nested paths.

API changes

None.