issue 4531 slice 99: PublishingShell set a legacy content list URL #5387

Parent: #4531

Slice goal

On Publishing → Design, an admin sets the legacy URL of one legacy content list. The name, description, and list type stay. A modern list does not offer this control. The URL shows only after a successful save.

Out of scope

Generator edits on modern lists (already shipped; those must keep leaving the legacy URL untouched). Location-scheme parameters. Delivery types.

Acceptance criteria

• Save writes the legacy URL only. Name, description, list type, and item filter stay. A blank or overlong URL does not write. Cancel does not write. HTTP 400, 403, and 409 leave the previous URL. Use the existing content-list update. If an omitted field would wipe the name or description, load the stored row and send those stored values unchanged.
• WebUI content lists + Vitest + H2 surface Playwright + product-docs/8.2/admin/publishing.md.
• Do not edit sitemanage-beans.xml, WebUI/src/main/ts/api/paths.ts, WebUI/src/main/ts/developer/messages.ts, DeveloperShell.tsx, deepLinks/allowlists.ts, product-docs/8.2/developer/index.md, product-docs/8.2/admin/index.md, or WebUI / perc-qa-automation package.json. Register nothing new in package.json; run H2 Playwright with npm run test:surface -- --path <spec>.
• Update parent [8.2] Unified publishing — next React increments #4531 Agent progress section.

Leave unassigned. Do not add In Progress.

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