CoreBluetooth: discover_services() can hang forever

[qdot]

Peripheral::discover_services() on CoreBluetooth can hang forever on several paths:

1. src/corebluetooth/internal.rs discover_services() (~L1494): no else branch — if the peripheral is not in the map (e.g. after disconnect removal or clear_peripherals), the future is never completed.
2. src/corebluetooth/central_delegate.rs didDiscoverServices (~L514): when error is Some, no event is sent, so the pending future is never resolved.
3. central_delegate.rs didDiscoverCharacteristicsForService (~L572): on error no event is sent, so that service never becomes discovered and check_discovered never fires.
4. A peripheral with zero services never calls check_discovered (only set_characteristics triggers it).

Fix direction: reply with an error for missing peripherals / delegate errors, and resolve immediately for an empty service set.