`ask(sources=[…])` scopes a question to some of the configured databases, carried
on the capability state so its search tool searches those. `Citation.source` names
the database a cited chunk came from, resolved from the search results the model
saw, which already carry it.
Context expansion routes each result through the database it came from: a
federating client has no repositories of its own.
The cite fallback, which looks up an id absent from this run's results, searches
only the selected databases. A chunk id says nothing about which database holds
it, so placing one means asking, and asking outside the selection would let a
question scoped to some databases cite another.
The loosely-specced client mocks in the capability tests now say they stand in for
a single-database client. A bare AsyncMock answers any attribute with a truthy
Mock, so `_federated` sent the fallback down the multi-database branch, and
`_source` reached a validated field.
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