fix(test): remove incompatible audio decode pipelines and fix GPU cleanup

Remove flac_decode, mp3_decode, and wav_decode test pipelines: the FLAC
decoder, MP3 decoder, and WAV demuxer all declare channels: 2 in their
static output pins, but the Opus encoder (the only audio encoder) only
accepts channels: 1. This static type mismatch causes pipeline validation
to reject the connection before any audio data flows.

Audio codec coverage is retained via opus_roundtrip (Ogg) and opus_mp4
(MP4) tests which exercise the full Opus encode/decode path.

Also fix GPU CI cleanup: use PID-based kill instead of pkill pattern
matching (the port number is in an env var, not the command line).

Signed-off-by: Devin AI <devin@streamkit.dev>
Signed-off-by: StreamKit Devin <devin@streamkit.dev>
Co-Authored-By: Claudio Costa <cstcld91@gmail.com>

Author: streamkit-devin

.github/workflows/e2e.yml

@@ -153,6 +153,7 @@ jobs:
           # instance that may be running on the self-hosted runner.
           SKIT_PORT=4546
           SK_SERVER__ADDRESS=127.0.0.1:${SKIT_PORT} ./target/debug/skit &
+          echo $! > /tmp/skit-gpu.pid
           # Wait for the server to be healthy
           HEALTHY=0
           for i in $(seq 1 30); do
@@ -175,7 +176,11 @@ jobs:
 
       - name: Stop skit server
         if: always()
-        run: pkill -f 'skit.*4546' || true
+        run: |
+          if [ -f /tmp/skit-gpu.pid ]; then
+            kill "$(cat /tmp/skit-gpu.pid)" 2>/dev/null || true
+            rm -f /tmp/skit-gpu.pid
+          fi
 
   e2e:
     name: Playwright E2E