feat: #3598964 Form-level rendering: states, wizard, source entity, prepopulation

Pieter Frenssenrequested to merge
issue/graphql_webform-3598964:3598964-rincewind into 3.x
  • Fix #states resolution: negated conditions (['checked' => FALSE]) now fold to the positive trigger/value, and states resolve across wizard pages.
  • Build the form for an optional source entity and apply Webform's query-string prepopulation via the new prepopulate argument on webformById, bubbling the URL / source-entity cache metadata into the response.
  • Expose WebformElementComputedBase (webform_computed_twig / _token) so source-entity token resolution can be asserted; wrap submission loading and confirmation in a captured RenderContext to keep computed-token cacheability from leaking.
  • Add wizard / prepopulate / source-entity test forms plus computed elements on the shared fixture, covered by new kernel tests.

Closes #3598964

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