feat(usage): plugin slot for Models tab extra columns

Lets enterprise plugins contribute per-route columns and a global
mount point on the Usage page's Models tab. Mirrors the existing
modelRoutes pattern: `usage.modelsExtraColumns` for cells,
`<PluginExtraFields name="UsageModelsPageGlobal" />` for the
page-level data lifecycle (receives the visible route ids so a
plugin can bulk-fetch per-row data in one call).

src/pages/usage/hooks/use-export-table.tsx

@@ -12,7 +12,11 @@ import useUsersColumns from './use-users-columns';
 
 type ColumnLike = {
   title: any;
-  dataIndex: string | string[];
+  // Plugin-contributed columns may omit `dataIndex` — they render via a
+  // bound component and aren't tied to a single row field. The export
+  // path skips those entries; an exported sheet has no place for a
+  // computed cell anyway.
+  dataIndex?: string | string[];
   key?: string;
 };