feat: --send-file for real audio testing + fix warnings

- --send-file <file.raw> sends a raw PCM file (48kHz mono s16le) through relay
- Combine with --record: --send-file talk.raw --record echo.raw <relay>
- Fixed all unused import warnings in echo_test.rs

Convert any audio to test format:
  ffmpeg -i input.mp3 -ar 48000 -ac 1 -f s16le input.raw

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-client/src/echo_test.rs

@@ -6,11 +6,8 @@
 
 use std::time::{Duration, Instant};
 
-use bytes::Bytes;
-use tracing::{debug, info, warn};
+use tracing::info;
 
-use wzp_proto::packet::{MediaHeader, MediaPacket};
-use wzp_proto::traits::{AudioDecoder, AudioEncoder, FecDecoder, FecEncoder};
 use wzp_proto::MediaTransport;
 
 use crate::call::{CallConfig, CallDecoder, CallEncoder};