fix(session): emit completion-only idle events (PoC)

[kitlangton]

Summary

• add reason to idle session status (completed | aborted | error)
• emit deprecated session.idle only for completed turns so interrupt/error flows no longer trigger completion listeners
• update cancellation and processor paths to set explicit idle reasons

Why this PoC

• this shows a backend-semantic fix to the double-sound issue without app-side timing suppression
• existing clients listening to session.idle now only hear true completion events

Testing

• not run in this environment (tsgo unavailable)
• sdk regeneration script also failed locally due missing typescript