Issue #3624679: Declare the default settings language and verify strict schema

Issue: https://www.drupal.org/project/mcp_sentinel/issues/3624679

Strict configuration installation fails because the default settings include a translatable dashboard broadcast message but do not declare a language. Add langcode: en to the shipped configuration and a kernel regression that explicitly enables schema constraint validation. The test preserves the translatable message definition.

Validation: reproduced the strict-schema failure before the fix; the regression passes with 1 test and 3 assertions on Drupal 10.6.17 / PHP 8.3, Drupal 11.3.17 / PHP 8.3, and Drupal 11.4.7 / PHP 8.4. Drupal and DrupalPractice coding standards pass. Existing dependency deprecation notices remain visible. Full CI is pending.

No update hook or schema changes: existing installations are not rewritten.