Take the database offline when a BuildContext context is disposed

[SimonCropp]

Fixes the failing `EfLocalDb.Tests.SimpleContext` on CI (run 2 on main).

`BuildContext` disposed its `SqlDatabase` before returning the context. With auto offline on (the default on CI), that took the database offline before the caller could use it, giving `SqlException 4060: Cannot open database ... Login failed`.

• `BuildContext` now hands the take-offline step to the returned context via a `TakeOfflineInterceptor`, which takes the database offline when the context's connection is disposed.
• `NewConnectionOwnedDbContext` keeps its public behaviour; the hand-over is an internal overload.
• Adds `BuildContext_AutoOffline`, which forces `dbAutoOffline: true` so it no longer depends on the CI environment. It fails without the fix.

EfClassicLocalDb has no `BuildContext`, so it is unaffected.