issue 4531 slice 90: PublishingShell add one parameter on a location scheme #5301

Parent: #4531

Slice goal

On PublishingShell Design, an admin adds one parameter (name, type, value) to one location scheme. The scheme name, generator, description, content type, and template stay. The new parameter is listed only after success.

Out of scope

Removing a parameter. Editing an existing parameter. Generator and description saves. Rename, create, copy, and delete.

Acceptance criteria

• Confirm adds that one parameter and leaves the other scheme fields unchanged. The parameter is listed only after success. WebUI Design + Vitest + H2 surface Playwright + product-docs/8.2/admin/publishing.md.
• Cancel does not write. An empty name or empty value does not write. A duplicate parameter name does not add a second row. HTTP 400, 403, and 409 leave the previous parameter list in place.
• Do not regress location-scheme rename.
• Update parent [8.2] Unified publishing — next React increments #4531 Agent progress section.

Leave unassigned. Do not add In Progress.

Co-Authored by Grok Build 1.0.46 using grok-4.7-build with agent night-issue-prs-triage.