[6.x] Connect element editor reactive layout refreshes

[riasvdv]

Description

Reactive controls in element editors now request server field-layout refreshes through refreshLayout. Root-scope responses update the root payload; nested-scope responses reconcile through the requesting renderer without replacing the root layout. Existing cancellation, baseline handling, nested-change refresh, and request context are preserved.

FormRenderer → refreshLayout → refreshForm
  root scope: update savedForm
  nested scope: return payload for scoped reconciliation

Extracted from #19665, anchor cc7b6f8. Includes the composable helper and renderer wiring together.

[github-actions]

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