fix: guard against stale SDP answers in wrong signaling state

Our own renegotiation answers can echo back through the relay and
get processed as if from the peer. Check that the peer connection
is in HAVE_LOCAL_OFFER state before applying a remote answer SDP,
preventing the "Called in wrong state: stable" error.

https://claude.ai/code/session_014espsysob7MrE8X8e75jFX

amethyst/src/main/java/com/vitorpamplona/amethyst/service/call/CallController.kt

@@ -40,6 +40,7 @@ import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 import org.webrtc.IceCandidate
+import org.webrtc.PeerConnection
 import org.webrtc.SessionDescription
 import org.webrtc.VideoFrame
 import org.webrtc.VideoSink
@@ -228,8 +229,18 @@ class CallController(
     }
 
     fun onCallAnswerReceived(sdpAnswer: String) {
-        Log.d(TAG) { "Answer received, SDP length=${sdpAnswer.length}, session=${webRtcSession != null}" }
-        webRtcSession?.setRemoteDescription(SessionDescription(SessionDescription.Type.ANSWER, sdpAnswer))
+        val session = webRtcSession ?: return
+        val signalingState = session.getSignalingState()
+        Log.d(TAG) { "Answer received, SDP length=${sdpAnswer.length}, signalingState=$signalingState" }
+
+        // An answer is only valid when we have a pending local offer.
+        // Ignore stale answers (e.g. our own renegotiation answer echoed back by the relay).
+        if (signalingState != PeerConnection.SignalingState.HAVE_LOCAL_OFFER) {
+            Log.d(TAG) { "Ignoring answer in $signalingState state (no pending local offer)" }
+            return
+        }
+
+        session.setRemoteDescription(SessionDescription(SessionDescription.Type.ANSWER, sdpAnswer))
         flushPendingIceCandidates()
     }
 

amethyst/src/main/java/com/vitorpamplona/amethyst/service/call/WebRtcCallSession.kt

@@ -263,6 +263,8 @@ class WebRtcCallSession(
         peerConnection?.addIceCandidate(candidate)
     }
 
+    fun getSignalingState(): PeerConnection.SignalingState? = peerConnection?.signalingState()
+
     fun setAudioEnabled(enabled: Boolean) {
         localAudioTrack?.setEnabled(enabled)
     }