External Dashboards API: expose container + tab fields #2150

Follow-up to #2015 (unified DashboardContainer with collapsible / bordered / tabbed options).

Why

The unified-group PR added three new persistent concepts to the dashboard document — containers[], per-container tabs[], and per-tile tabId — but the external API (packages/api/src/routers/external-api/v2/dashboards.ts) and its OpenAPI spec haven't been audited to confirm these round-trip correctly through create / update / get. External integrations that programmatically build dashboards currently can't take advantage of the new organization, and older automations may silently drop fields they don't understand.

Scope

• Verify POST /dashboards and PUT /dashboards/:id accept the new schema shape (containers, tabs, containerId, tabId, collapsible, bordered).
• Verify GET /dashboards/:id returns containers + tabs fields (not silently stripped by a transform layer).
• Update OpenAPI spec (packages/api/openapi.json) so the DashboardContainer / DashboardContainerTab / Tile.tabId schemas are documented and generated SDKs pick them up.
• Add an integration test: create → read → update a dashboard with a multi-tab container and assert field preservation.
• Document any legacy type: 'section' handling (spec should note the field is accepted but stripped).

Non-goals

• Changes to UI behavior (covered by #2015).
• External API auth / permissioning changes.

Parent

HDX-3729 (Dashboard: Sections, groups with tabs, and panel organization).