Add call.accept() to the Relay Call API

[jakubkarolczyk]

Summary

Adds call.accept() to the Relay Call API — send a SIP provisional response (180 Ringing, or 183 Session Progress via early_media=True) on an inbound call without answering it. The call stays unanswered and unbilled.

async def on_call_received(call):
    await call.accept()                    # 180 Ringing
    # or: await call.accept(early_media=True)   # 183 Session Progress
    # ...run checks, then connect/answer

Why

Related to signalwire/cloud-product#20389. When a Relay app deliberately does not answer() an inbound call (e.g. it runs authorization checks and only answers once a destination picks up), SignalWire sends the upstream carrier no provisional response; the carrier hits its post-dial-delay limit, cancels, and re-attempts. accept() lets the app send a 180/183 to satisfy the carrier's PDD timer without committing to an answer.

Server method: signalwire/mod_infrastructure#1846.

Changes

• relay/call.py — accept(early_media=False, **kwargs), mirroring answer (dispatches calling.accept with early_media).
• tests/unit/relay/test_call.py — test_accept (asserts the calling.accept wire call + early_media).
• relay/docs/call-methods.md — accept under Lifecycle.
• CHANGELOG.md — [Unreleased] entry.