Replace generic mockProvider with modelRecordingProvider that captures
the model parameter passed to Chat(). After delegation from alpha to
beta, assert the recorded model is "model-beta" — proving the child
turn actually ran with the target agent's configuration, not the
caller's.
Also add wiring tests:
- TestDelegateToolNotRegistered_SingleAgent: single-agent has no
delegate in its tool registry
- TestDelegateToolRegistered_MultiAgent: both agents in a two-agent
setup have the delegate tool
Ref: #2148
xiaoen
a34120b821