feat(amd): answering machine detection

[chenghao-mou]

Placeholder issue for livekit/agents#4906

[KrishnaShuk]

Since the core AMD logic has been successfully ported into JS (agents/src/voice/amd.ts) and all recent tunable/parity PRs (#1368, #1394, #1414) have either merged or finalized tracking, the scope of this placeholder has been met.

This issue can proactively be closed as completed.