GET /api/providers/custom-endpoints read only providers:, so a post-migration
custom_providers: list entry (still routed by get_compatible_custom_providers)
had no row and could be deleted nowhere. Build the legacy rows from that same
merged view (source "custom_providers"; entries from providers: carry a
provider_key, legacy ones do not). DELETE removes the matching list entry
when the id is not under providers:; activate promotes the entry to
providers.<key> first, since the main slot names providers by key.
The doctor residue check keeps firing but no longer claims the row is missing;
its rationale, the docs line and the non-list message now talk about the
retired list store ("legacy custom_providers entries are ignored until it is").
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