Add call.ring() to the Relay Call API

[jakubkarolczyk]

Summary

Adds call.ring() to the Relay Call API — send 180 Ringing on an inbound call without answering it. Signalling only: no media, no answer, no billing.

async def on_call_received(call):
    await call.ring()          # 180 Ringing to the caller
    # ...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. ring() lets the app send a 180 to satisfy the carrier's PDD timer without committing to an answer.

Server method: signalwire/mod_infrastructure#1846.

Behavior worth knowing

• Ringing a call takes control of it, like answer() — it is no longer offered to other consumers and can no longer be handed off with pass.
• Because taking control disables the inbound-offer timeout, the server bounds a rung call with its own hold timeout: if it is never answered it is hung up with SIP 480 (default 180s).
• On WebRTC legs there is no upstream SIP carrier, so ring is a no-op that returns success without sending a provisional.

Changes

• relay/call.py — ring(**kwargs), mirroring answer (dispatches calling.ring).
• tests/unit/relay/test_call.py — test_ring (asserts the calling.ring wire call).
• relay/docs/call-methods.md — ring under Lifecycle.
• CHANGELOG.md — [Unreleased] entry.

Note on the earlier revision

This PR originally added accept(early_media=False), sending either 180 or 183. The server-side method was subsequently cut down to a single, media-free ring that sends 180 only — the 183/early-media path is deferred to a separate preanswer method. This PR has been updated to match: the method is ring, and early_media is gone. The server's request validator is a strict allowlist, so the previous form would have been rejected with a 400.