Add early validation in OnConfigurationChange to verify the configured
channel exists in Mattermost and the Shelfmark server is reachable,
giving admins immediate feedback instead of discovering issues at runtime.
- Add Ping() method to shelfmark.Client for lightweight reachability check
- Add validateConfiguration() on Plugin that checks channel + Shelfmark
- Add i18n messages for channel not found and Shelfmark unreachable
- Add unit tests for Ping and validateConfiguration
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Felipe M.
a4ee9379eb