issue 4531 slice 89: PublishingShell set a location scheme description #5300

Parent: #4531

Slice goal

On PublishingShell Design, an admin changes only the description of one location scheme and saves. Blank clears the description. The scheme name, generator, content type, template, and parameters stay. The description is shown only after success.

Out of scope

Generator (slice 88). Rename. Parameters. Create, copy, and delete.

Acceptance criteria

• A description-only save, including a blank clear, leaves every other field unchanged. The stored description is shown only after success. WebUI Design + Vitest + H2 surface Playwright + product-docs/8.2/admin/publishing.md.
• Cancel does not write. HTTP 400, 403, and 409 leave the previous description in place.
• Do not regress location-scheme rename or generator save.
• Update parent [8.2] Unified publishing — next React increments #4531 Agent progress section.

Leave unassigned. Do not add In Progress.

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