test(network): unit tests for ticket codec + gossip wire types

Add #[cfg(test)] coverage for the join-link codec and gossip wire format:
PeerSpeakTicket Display/FromStr round-trip and — the real gap —
malformed-input rejection (empty / non-base64 / valid-base64-non-JSON all
return NetError::InvalidTicket, never panic, since users paste untrusted
text here); PeerState serde round-trip; and GossipMessage::Leave /
Announce + GossipPayload round-trips. Tests-only; no production change.

Implemented by Gemini (junior implementer), reviewed and verified by senior
(cargo build + clippy --all-targets + cargo test all green).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/network/gossip.rs

@@ -285,3 +285,53 @@ impl RoomState for IrohGossipState {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::network::PeerState;
+    use iroh::SecretKey;
+
+    fn sample_peer_state() -> PeerState {
+        let secret = SecretKey::generate();
+        let public = secret.public();
+        let addr = iroh::EndpointAddr::from(public);
+        PeerState {
+            name: "TestPeerGossip".to_string(),
+            is_muted: true,
+            addr,
+        }
+    }
+
+    #[test]
+    fn test_gossip_message_leave_round_trip() {
+        let original = GossipMessage::Leave;
+        let serialized = serde_json::to_string(&original).unwrap();
+        let deserialized: GossipMessage = serde_json::from_str(&serialized).unwrap();
+        assert!(matches!(deserialized, GossipMessage::Leave));
+    }
+
+    #[test]
+    fn test_gossip_payload_announce_round_trip() {
+        let peer_state = sample_peer_state();
+        let author = peer_state.addr.id;
+        let payload = GossipPayload {
+            author,
+            msg: GossipMessage::Announce(peer_state.clone()),
+        };
+
+        let serialized = serde_json::to_string(&payload).unwrap();
+        let deserialized: GossipPayload = serde_json::from_str(&serialized).unwrap();
+
+        assert_eq!(deserialized.author, author);
+        match deserialized.msg {
+            GossipMessage::Announce(state) => {
+                assert_eq!(state, peer_state);
+            }
+            GossipMessage::Leave => {
+                panic!("Expected GossipMessage::Announce, got Leave");
+            }
+        }
+    }
+}
+