* Fix Magentic to share agent replies across team
The per-round instruction was sent untargeted (fan-out delivered it to
every participant) and replies were never relayed, so a later speaker saw
the prior speaker's instruction but not its response - inverted from
GroupChatHost and the Python reference.
- Target the instruction at the selected speaker only.
- Broadcast each reply to the other participants (buffered, no TurnToken),
excluding the responder via _currentSpeakerExecutorId, mirroring
GroupChatHost.
- Persist _currentSpeakerExecutorId across checkpoints.
- Add a regression test.
* Address review feedback: null-guard, explicit checkpoint key, drop vacuous assertion
* Address review feedback: centralize checkpoint keys, clear current speaker
- Move CurrentSpeakerStateKey into MagenticConstants as
nameof(CurrentSpeakerStateKey)
- Clear _currentSpeakerExecutorId in ResetAndReplanAsync and
PrepareFinalAnswerAsync so a checkpoint taken in those windows does not
persist a stale speaker
- Add UTF-8 BOM to RecordingEchoAgent.cs to satisfy the format check.
Hasan Ghomi