test: cover chat wire type + history cap

GossipMessage::Chat serde round-trips (normal, empty strings, u64::MAX ts,
unicode/emoji), GossipPayload{Chat} round-trip, and push_chat history-cap
behaviour (single, below cap order-preserved, above cap drops oldest keeping
the newest CHAT_HISTORY_MAX in order). Gemini, senior-audited.

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

src/app/mod.rs

@@ -1553,4 +1553,65 @@ mod tests {
             Some(Sound::Reconnected)
         );
     }
+
+    #[test]
+    fn test_push_chat_single() {
+        use super::{push_chat, ChatEntry};
+        let mut messages = Vec::new();
+        let entry = ChatEntry {
+            name: "Alice".to_string(),
+            text: "Hello".to_string(),
+            mine: true,
+        };
+        push_chat(&mut messages, entry);
+        assert_eq!(messages.len(), 1);
+        assert_eq!(messages[0].name, "Alice");
+        assert_eq!(messages[0].text, "Hello");
+        assert!(messages[0].mine);
+    }
+
+    #[test]
+    fn test_push_chat_below_cap() {
+        use super::{push_chat, ChatEntry, CHAT_HISTORY_MAX};
+        let mut messages = Vec::new();
+        for i in 0..CHAT_HISTORY_MAX - 10 {
+            push_chat(
+                &mut messages,
+                ChatEntry {
+                    name: format!("User{}", i),
+                    text: format!("Msg{}", i),
+                    mine: i % 2 == 0,
+                },
+            );
+        }
+        assert_eq!(messages.len(), CHAT_HISTORY_MAX - 10);
+        assert_eq!(messages[0].name, "User0");
+        assert_eq!(messages[0].text, "Msg0");
+        assert_eq!(messages[messages.len() - 1].name, format!("User{}", CHAT_HISTORY_MAX - 11));
+        assert_eq!(messages[messages.len() - 1].text, format!("Msg{}", CHAT_HISTORY_MAX - 11));
+    }
+
+    #[test]
+    fn test_push_chat_above_cap() {
+        use super::{push_chat, ChatEntry, CHAT_HISTORY_MAX};
+        let mut messages = Vec::new();
+        let total_pushes = CHAT_HISTORY_MAX + 5;
+        for i in 0..total_pushes {
+            push_chat(
+                &mut messages,
+                ChatEntry {
+                    name: format!("User{}", i),
+                    text: format!("Msg{}", i),
+                    mine: i % 2 == 0,
+                },
+            );
+        }
+        assert_eq!(messages.len(), CHAT_HISTORY_MAX);
+        // The first 5 should be dropped. First remaining should be index 5.
+        assert_eq!(messages[0].name, "User5");
+        assert_eq!(messages[0].text, "Msg5");
+        // The last remaining should be index total_pushes - 1.
+        assert_eq!(messages[messages.len() - 1].name, format!("User{}", total_pushes - 1));
+        assert_eq!(messages[messages.len() - 1].text, format!("Msg{}", total_pushes - 1));
+    }
 }