fix(amd): start SIP detection after answer

[dexhunter]

Summary

• start the AMD detection budget when an outbound SIP call becomes active, rather than when its early-media track is subscribed
• add a separate configurable sip_answer_timeout guard, defaulting to 60 seconds, for calls that never become active
• keep non-SIP behavior unchanged and add lifecycle, timeout, fallback, and public-export coverage

Fixes #6187.

Why

Carrier early media can subscribe the audio track well before answer. On the current implementation, that consumes the default 20-second detection_timeout during ringback and can emit uncertain / detection_timeout before a late-answering person or voicemail becomes active.

This change preserves detection_timeout as a post-answer AMD budget. The new answer guard independently bounds calls that never reach active.

Evaluation

The deterministic harness models a SIP track subscribed at t=0 and a call becoming active at t=32s:

Metric	main	This PR
detection_timeout_pre_answer_ms	20,000 ms	0 ms

The metric is virtual-time based, so it is independent of CPU speed. The strict harness also checks the default and custom answer deadlines, duplicate timer starts, task cancellation, non-SIP behavior, legacy fallback, and all classifier verdict paths.

Validation

• Python 3.10: 27 focused AMD tests passed
• Python 3.14: 27 focused AMD tests passed
• make check: formatting, lint, and mypy passed across 617 source files
• strict evaluator: detection_timeout_pre_answer_ms=0

The deferred-timer and separate-answer-guard direction was identified through autoresearch with Weco. The final patch was reduced and hardened manually against lifecycle and compatibility counterexamples.

[CLAassistant]

All committers have signed the CLA.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.