test: FailUpload must not overwrite a finished upload; ffmpeg stderr is bounded, red (review of #55)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/adapters/audio/ffmpeg_test.go

@@ -126,3 +126,15 @@ printf data > "$last"`)
 	assert.Equal(t, "32", args[ms+1])
 	assert.Less(t, ms, in, "an input option placed after -i is applied to the output")
 }
+
+func TestFFmpegToWAV_ABoundedPartOfAFloodingStderrIsKept(t *testing.T) {
+	// A hostile mp3 made ffmpeg print ~0.4 bytes of stderr per input byte (83 MB for a
+	// 209 MB file), all of it buffered, then copied twice into the error (#49 review).
+	bin := stub(t, `head -c 5000000 /dev/zero | tr '\0' 'e' >&2; exit 1`)
+
+	err := (&audio.FFmpeg{Bin: bin}).ToWAV(context.Background(), "/spool/in", filepath.Join(t.TempDir(), "o.wav"))
+
+	require.Error(t, err)
+	assert.Less(t, len(err.Error()), 16*1024, "the error carries a bounded head of stderr, not all of it")
+	assert.Contains(t, err.Error(), "eeee", "and still carries some of it")
+}