Fix 500 when creating text widgets without visualization_id

[amirshahzadhashmi7145]

Summary

• POST /api/widgets no longer 500s when visualization_id is omitted (text widgets).
• Fixes WidgetListResource.post() raises unhandled KeyError (500) when visualization_id is omitted for text widgets #7793

Test plan

• POST /api/widgets with {dashboard_id, width, text} (no visualization_id) returns 200
• Same request with "visualization_id": null still returns 200
• Creating a visualization widget still works

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Allows text widgets to be created without a visualization.

The PR appears safe to merge.

Summary

The PR allows text widgets to be created when visualization_id is omitted rather than explicitly set to null.

• Defaults a missing visualization_id to None in the widget creation handler.
• Adds API coverage for creating a text widget without the key.

_{Reviews (4) · Last reviewed commit: "Merge branch 'master' into fix/7793-text..." · Reviewed by Greptile}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}