fix(media-server): fall back to sole agent peer when peer_id missing

Rails' /agent_answer forwards only the SDP answer to the media server; it
doesn't persist or echo the peer_id returned from /agent-offer. The
AgentAnswer handler required peer_id and 400'd on every Rails call, so
Peer B never completed its handshake. With no agent remote track, the
bridge delivered nothing to Meta, and Meta tore the call down after ~20s
with error 138021 ("WhatsApp client terminated the call due to not
receiving any media").

Add Session.SoleAgentPeerID and have the handler use it when peer_id is
absent. Single-agent sessions now complete negotiation without extra
Rails plumbing. Multi-agent sessions still require an explicit peer_id.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

enterprise/media-server/internal/server/handlers.go

@@ -297,9 +297,16 @@ func (h *Handlers) AgentAnswer(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, "sdp_answer is required")
 		return
 	}
+	// When peer_id is omitted (single-agent sessions), fall back to the only
+	// agent peer attached to the session. Rails doesn't currently surface
+	// peer_id through ActionCable, so browsers just send the SDP answer.
 	if req.PeerID == "" {
-		writeError(w, http.StatusBadRequest, "peer_id is required")
-		return
+		if only, ok := sess.SoleAgentPeerID(); ok {
+			req.PeerID = only
+		} else {
+			writeError(w, http.StatusBadRequest, "peer_id is required (session has multiple agent peers)")
+			return
+		}
 	}
 
 	if err := sess.SetAgentAnswer(req.PeerID, req.SDPAnswer); err != nil {

enterprise/media-server/internal/session/session.go

@@ -204,6 +204,22 @@ func (s *Session) CreateAgentPeer(peerID string, role peer.PeerRole, iceServers
 	return sdpOffer, nil
 }
 
+// SoleAgentPeerID returns the peer_id when the session has exactly one agent
+// peer. Used by the agent-answer handler as a fallback for clients that don't
+// track peer ids.
+func (s *Session) SoleAgentPeerID() (string, bool) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if len(s.AgentPeers) != 1 {
+		return "", false
+	}
+	for id := range s.AgentPeers {
+		return id, true
+	}
+	return "", false
+}
+
 // SetAgentAnswer sets the agent browser's SDP answer on the specified agent
 // peer, completing the WebRTC handshake.
 func (s *Session) SetAgentAnswer(peerID, sdpAnswer string) error {