Add descriptions to schemas for all catalog items to improve generation quality

_↴ Ported from [flutter/genui#554](https://github.com/flutter/genui/issues/554) — originally opened by [jacobsimionato](https://github.com/jacobsimionato) on 2025-11-18._
_Original labels: P2, front-line-handled, second-line-triaged, quality_

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Branched from https://github.com/flutter/genui/issues/448

This is a stop-gap measure, because these descriptions will actually need to be added at the A2UI level and then eventually copied to Gen UI. But it's worth us still improving these slightly where we can.

@victoreronmosele has a PR out for this at https://github.com/flutter/genui/pull/498

Look at the linked PR #498 in the flutter/genui repository to understand the pattern for adding descriptions to schemas. Identify the catalog item schemas in the a2ui codebase, likely in a directory like `schemas/` or `src/schemas/`. Add clear, descriptive text to each schema's definition. Verify the changes do not break existing schema validation tests.

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