New CommissioningComplete code does not ensure there is a working CASE session

[bzbarsky-apple]

Problem

The DeviceCommissioner::CommissioningComplete function added in #9041 uses GetDevice, not GetConnectedDevice, so does not ensure that there is an actual CASE session to the given node id.

Proposed Solution

Fix that. Though see #9480 about whether this function should exist in the first place.

And maybe rename GetDevice to GetMaybeDisconnectedDevice to make it clearer what it does?

@sweetymhaiske @pan-apple

[sweetymhaiske]

@bzbarsky-apple CommissioningComplete function gets called after OperationalDiscoveryComplete function. In OperationalDiscoveryComplete function GetConnectedDevice API is called. Hence CommissioningComplete API will be called only after successful CASE session.

[bzbarsky-apple]

CommissioningComplete function gets called after OperationalDiscoveryComplete function

This requirement is not documented in the API documentation for CommissioningComplete, and is relying on implementation details of other functions that might change, no? This is basically a bug waiting to happen.

Though again, this function should not exist.