issue 4532 slice 82: EditorHost leave a schema read-only field unchanged

[natechadwick-intsof]

Parent: #4532

Slice goal

In EditorHost edit mode, a field the content type marks read-only cannot be edited. Saving a different field does not change the read-only value. Reload shows the previous read-only value.

Out of scope

View mode, which is already read-only for every field. Required-field refusals. File and image widgets. Making a read-only field writable.

Acceptance criteria

• The read-only control is not editable. Save of a writable field succeeds, and reload keeps the read-only value. The fields PUT omits that read-only field, or sends only the original stored value, never a client edit. WebUI + Vitest + H2 surface Playwright + product-docs/8.2/admin/content-explorer.md (append under the editor section only; do not rewrite the Active Assembly section).
• Cancel does not write. HTTP 400, 403, and 409 do not claim success and do not change the read-only value.
• Do not edit WebUI/package.json, WebUI/src/main/frontend/package.json, or modules/perc-qa-automation/frontend/package.json (open dependabot PRs build(deps-dev): bump the npm-development group across 4 directories with 4 updates #5288 and build(deps): bump the npm-production group across 2 directories with 1 update #5283). Add a path-filtered Playwright spec only.
• Update parent [8.2] React content editor — next product increments #4532 Agent progress section.

Leave unassigned. Do not add In Progress.

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