Issue #3593113 by mably: [3.0.x backport] Per-config checkboxes on the override delete confirmation form

Frank Mablyrequested to merge
issue/domain-3593113:3593113-delete-checkboxes-3.0.x into 3.0.x

3.0.x backport of !395 (merged) (#3593113 follow-up).

Brings the delete-confirmation rework to 3.0.x: per-config checkboxes when several configs are listed, a direct confirm with no checkbox for a single override, collapsible per-config review sections (expanded by default), a "Not overridden for this domain." note for registered configs that carry no values, and an empty-selection guard.

The DeleteForm change is identical to 3.x (only the unrelated @internal docblock, absent on 3.0.x, was dropped). Updates the existing DomainConfigUiSavedConfigTest for the new copy and adds DomainConfigUIDeleteFormTest.

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