fix(frontend): Open variation editor modal via route registration

[mattbrailsford]

Summary

Closes #144. Selecting a document type in the Entity Context picker on a test variation silently did nothing.

The variation config editor modal was opened via modalManager.open(), so <umb-modal> mounted an UmbContextBoundary over UMB_ROUTE_CONTEXT instead of a real <umb-router-slot>. Inside the modal, uai-mock-entity (added in #125) could no longer consume UMB_ROUTE_CONTEXT, its UmbModalRouteRegistrationController never fired, and #editorRoutePath never resolved — so the sub-type click pushed nothing.

This switches the variation builder to UmbModalRouteRegistrationController + history.pushState, mirroring uai-mock-entity's own pattern. The modal now carries a router, publishes UMB_ROUTE_CONTEXT to its content, and the chain that #125 needs for nested block-grid editors stays intact.

UaiTestVariationConfigEditorModalValue gets | undefined added for the same reason UaiMockEntityEditorModalValue does — UmbModalRouteSetupReturn's typing only makes value optional when the token's value type is ... | undefined.

Test plan

• Test → variation → Entity Context → +Add → Document → Article — the mock entity editor should now open
• Inside the entity context editor, exercise a doc-type with block grid / block list / block rte properties — "Create new" should still open the catalogue modal (not silently insert), and block edit links should still work (regression check for Route-register mock entity editor modal for block grid support #125)
• Add a new variation, edit an existing variation, cancel via Escape, cancel via the Cancel button — variations array should reflect the action correctly
• After submit/cancel, hit browser back/forward — the variation modal should not re-open into a stale Add state

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