issue 4530 slice 104: Assembly host save one date field

[natechadwick-intsof]

Parent: #4530

Slice goal

On the Active Assembly host, an author saves one calendar date on a date field of the assembled page. Reload shows that date. This is not a datetime and not the Content Editor host.

Out of scope

Number (slice 103). Required-blank number (slice 105). Datetime, clear, and EditorHost date save.

Acceptance criteria

• Save writes that one date through the existing item field save. Reload of the assembly host shows the same date. Other fields stay as they were. WebUI assembly host + Vitest + H2 surface Playwright + append only under the assembly section of product-docs/8.2/admin/content-explorer.md.
• A blank date does not wipe a stored date. Cancel, a read-only date, and HTTP 400, 403, and 409 do not claim success.
• Do not edit api/paths.ts, DeveloperShell.tsx, sitemanage-beans.xml, or WebUI / perc-qa-automation package.json.
• Update parent [8.2] Content Explorer — next product increments #4530 Agent progress section.

Leave unassigned. Do not add In Progress.

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

[natechadwick-intsof]

night-issue-prs: starting work

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

[natechadwick-intsof]

night-issue-prs: opened #5373

Active Assembly saves one calendar date through the item field PUT. A blank date does not clear a stored date. Cancel, a read-only date, and HTTP 400/403/409 do not claim success. H2 Playwright surface: 7 passed.

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