Add TestConnectionResponse DTO and From conversion. Implement POST
/federation/providers/{id}/test-connection handler, route and utoipa
docs and wire it to the federation service.

Update frontend API client and add useTestUserFederationConnection
mutation. Refactor LDAP create/detail pages to use a nested config
shape, parse connection URLs, base64-encode bind passwords on save,
decode for display, and add a test-connection action. Add confetti hook
and canvas-confetti deps and update components registry.

Add sync-users POST handler, route and OpenAPI docs plus
SyncUsersResponse DTO. Wire federation sync through application services
and repository (get_mapping_by_user_id). Extend SyncResult with
start/completion timestamps and map to DTO. Frontend: add useSyncUsers
mutation, invalidate provider queries and add sync UI/button. Security:
reject empty LDAP bind passwords. Add Federated credential variant in
credential entities.