fix(rtp): install negotiated DTMF payload type

[ryanmurf]

Summary

• remove the implicit RTP telephone-event PT 101 fallback and represent DTMF as disabled until SDP negotiates it
• install the accepted dynamic telephone-event mapping on inbound calls and update existing outbound RTP sessions from the 2xx SDP answer
• use the negotiated mapping for both RFC 4733 receive detection and transmit packets
• clear DTMF support when the remote SDP does not negotiate telephone-event

RFC 4733 section 2.1 says telephone-event has no static payload type and uses a payload number established dynamically out of band; section 2.4.1 defines its SDP rtpmap relationship: https://www.rfc-editor.org/rfc/rfc4733#section-2.1 and https://www.rfc-editor.org/rfc/rfc4733#section-2.4.1

Regression coverage

• parses an SDP audio mapping with telephone-event PT 110 and selects 110 rather than the local default PT
• installs PT 110 on a live RTP session, sends identical RFC 4733 end-event payloads on PT 101 and PT 110, proves PT 101 remains a voice frame, and proves the receiver detects PT 110 as digit 5 with a 100 ms duration

Validation

• cargo +1.97.0 clippy -p asterisk-sip --all-targets -- -D warnings
• cargo +1.97.0 clippy --workspace --exclude pjsip-shim -- -D warnings
• cargo +1.97.0 test --workspace --exclude pjsip-shim

Fixes #72