Fix G.722 ADPCM state reset between TTS chunks causing robotic audio

[Copilot]

G.722 TTS output sounded robotic because each send_rtp_audio() call created a fresh av.AudioCodecContext, resetting the ADPCM predictor at every chunk boundary. The receiver decoded a series of independent ADPCM streams rather than one continuous one.

Changes

• voip/codecs/base.py: Add PayloadEncoder protocol, PerPacketEncoder (stateless delegate to codec.packetize), and RTPCodec.create_encoder() factory — mirrors the existing decoder pattern
• voip/codecs/g722.py: Add G722Encoder dataclass with a persistent av.AudioCodecContext that survives across packetize() calls; add G722.create_encoder(). Encoding is eager (not lazy generator) so ADPCM state and PTS advance immediately on each call
• voip/audio.py: Add payload_encoder: PayloadEncoder to AudioCall, initialized in __post_init__ via codec.create_encoder(), used in send_rtp_audio() replacing self.codec.packetize()
• docs/codecs.md: Document G722Encoder, PayloadEncoder, PerPacketEncoder

# Before: fresh codec context → ADPCM reset every chunk
for chunk in tts_chunks:
    await self.send_rtp_audio(chunk)  # G722.encode() → new AudioCodecContext each time

# After: persistent context across all chunks for this call
self.payload_encoder = G722.create_encoder()  # one G722Encoder per call
for chunk in tts_chunks:
    await self.send_rtp_audio(chunk)  # G722Encoder.packetize() → state preserved

Original prompt

---

This section details on the original issue you should resolve

<issue_title>🐛 G.722 is sounding off</issue_title>
<issue_description>### Bug Description

When I use the echo call, the response is audible but broken. It seems too short and sounds "like a robot". The pitch and gain seem correct.

Expected Behavior

I should hear my voice in the echo call clear as day, since it's a high-res call and I have a wired broadband connection to the UAS. My ping stats are round-trip min/avg/max/stddev = 26.084/29.938/32.507/2.109 ms.

Version

0.2.1

Stack trace

No response

Steps to Reproduce

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

@codingjoe #43 worked for the echo call, but the agent suffers from the same strange sound. Open a new PR against main and check if the TTS output is correctly send via G.722

Custom agent used: SuperJoe
CodingJoe's digital clone following his coding guidelines and best practices.

• Fixes 🐛 G.722 is sounding off #42

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.36%. Comparing base (53b1491) to head (87410c3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   94.25%   94.36%   +0.10%     
==========================================
  Files          24       24              
  Lines        1759     1793      +34     
==========================================
+ Hits         1658     1692      +34     
  Misses        101      101              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codingjoe]

That wasn't it 👎