fix(capture): derive video ids from pipelines and guard empty list (#711)

when NEKO_CAPTURE_VIDEO_PIPELINES is set without capture.video.ids the
VideoIDs list stayed empty, so capture.Video().IDs() returned [] and the
first websocket signal request crashed the whole server with
"index out of range [0] with length 0" in signalRequest.

now we fill VideoIDs from the configured pipeline keys (sorted by id)
whenever no ids are set explicitly, and signalRequest returns a plain
error instead of panicking when there is no video stream at all.

added tests for the config loader, the capture manager and the signal
handler so this path stays covered.

server/internal/websocket/handler/signal_test.go

@@ -0,0 +1,117 @@
+package handler
+
+import (
+	"testing"
+
+	"github.com/pion/webrtc/v4"
+
+	"github.com/m1k1o/neko/server/pkg/types"
+	"github.com/m1k1o/neko/server/pkg/types/message"
+)
+
+// The fakes below embed the real interfaces so we only implement the methods
+// signalRequest actually calls.
+type fakeSession struct {
+	types.Session
+	profile types.MemberProfile
+}
+
+func (s *fakeSession) ID() string { return "test-session" }
+
+func (s *fakeSession) Profile() types.MemberProfile { return s.profile }
+
+func (s *fakeSession) PrivateModeEnabled() bool { return false }
+
+func (s *fakeSession) Send(string, any) {}
+
+type fakeWebRTC struct {
+	types.WebRTCManager
+	peer *fakePeer
+}
+
+func (w *fakeWebRTC) ICEServers() []types.ICEServer { return nil }
+
+func (w *fakeWebRTC) CreatePeer(types.Session) (*webrtc.SessionDescription, types.WebRTCPeer, error) {
+	w.peer = &fakePeer{}
+	return &webrtc.SessionDescription{SDP: "fake-offer"}, w.peer, nil
+}
+
+type fakePeer struct {
+	types.WebRTCPeer
+	video types.PeerVideoRequest
+}
+
+func (p *fakePeer) SetPaused(bool) error { return nil }
+
+func (p *fakePeer) SetVideo(v types.PeerVideoRequest) error {
+	p.video = v
+	return nil
+}
+
+func (p *fakePeer) Video() types.PeerVideo { return types.PeerVideo{} }
+
+func (p *fakePeer) SetAudio(types.PeerAudioRequest) error { return nil }
+
+func (p *fakePeer) Audio() types.PeerAudio { return types.PeerAudio{} }
+
+type fakeCapture struct {
+	types.CaptureManager
+	ids []string
+}
+
+func (c *fakeCapture) Video() types.EncodedStreamSelector {
+	return &fakeSelector{ids: c.ids}
+}
+
+type fakeSelector struct {
+	types.EncodedStreamSelector
+	ids []string
+}
+
+func (s *fakeSelector) IDs() []string { return s.ids }
+
+func newSignalTestHandler(capture types.CaptureManager) (*MessageHandlerCtx, *fakeWebRTC) {
+	webrtcManager := &fakeWebRTC{}
+	h := &MessageHandlerCtx{
+		capture: capture,
+		webrtc:  webrtcManager,
+	}
+	return h, webrtcManager
+}
+
+func watchSession() *fakeSession {
+	return &fakeSession{profile: types.MemberProfile{CanWatch: true}}
+}
+
+// On master, an empty video id list made signalRequest index videos[0] and
+// panic the whole server. It must return a descriptive error instead.
+func TestSignalRequestNoVideoStreams(t *testing.T) {
+	h, _ := newSignalTestHandler(&fakeCapture{ids: nil})
+
+	payload := &message.SignalRequest{}
+	if err := h.signalRequest(watchSession(), payload); err == nil {
+		t.Fatal("expected an error when no video streams are configured, got nil")
+	}
+}
+
+// When streams exist and the client sends no selector, the first video id is
+// used as the default stream.
+func TestSignalRequestDefaultsToFirstVideo(t *testing.T) {
+	h, webrtcManager := newSignalTestHandler(&fakeCapture{ids: []string{"high", "low", "medium"}})
+
+	payload := &message.SignalRequest{}
+	if err := h.signalRequest(watchSession(), payload); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	selector := webrtcManager.peer.video.Selector
+	if selector == nil {
+		t.Fatal("expected a default selector to be set")
+	}
+	if selector.ID != "high" {
+		t.Fatalf("expected default video id %q, got %q", "high", selector.ID)
+	}
+	if selector.Type != types.StreamSelectorTypeExact {
+		t.Fatalf("expected exact selector type, got %v", selector.Type)
+	}
+}