feat(ami): expose per-call RTP proof counters

[ryanmurf]

Summary

• count successfully transmitted and received non-empty voice frames per RTP session
• count logical RFC 4733 digits on successful transmit and receiver detection after repeated-end deduplication
• include DTMF reliability packets in the existing packet/octet totals
• associate counters with each SIP Channel and Uniqueid
• retain the last 256 completed-call records so proof remains queryable after hangup
• add an authenticated AMI RTPStats action with active/completed state and TX/RX packet, octet, voice-frame, and DTMF-digit counters

Regression coverage

• drives real UDP RTP through the SIP channel driver in both directions
• receives one non-empty voice frame and transmits one non-empty voice frame
• sends three identical RFC 4733 end packets and proves RTPDTMFDigitsRx is exactly 1
• transmits one logical DTMF digit as six RFC 4733 reliability packets and proves RTPDTMFDigitsTx is exactly 1
• proves AMI reports RTPPacketsTx=7, RTPPacketsRx=4, and all four logical proof counters as 1
• hangs up the driver channel and proves the same counters remain queryable by Uniqueid with RTPActive=false
• proves empty voice payloads increase packet totals but never voice-frame proof counters

Protocol semantics

• RFC 3550 section 6.4.1 defines sender packet/octet accounting: https://www.rfc-editor.org/rfc/rfc3550.html#section-6.4.1
• RFC 4733 section 2.5.1.4 specifies repeated final event reports for reliability: https://www.rfc-editor.org/rfc/rfc4733.html#section-2.5.1.4

Validation

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

Fixes #78