Omit null media in history frames (schema-conformant)

history() wrote "media": null for streaming finals, but the frame
schema says array. The app's HistoryMessage.media was non-nullable, so
deserialization threw and every history page was silently dropped.
Omit the key when media is absent instead.

gateway-plugin/outbox.py

@@ -207,21 +207,25 @@ class Outbox:
                 role = payload.get("role")
                 if role not in ("user", "assistant"):
                     continue
-                final.append(
-                    {
-                        "cursor": int(r["cursor"]),
-                        "message_id": payload.get("message_id"),
-                        "role": role,
-                        "text": payload.get("text", ""),
-                        "reasoning": payload.get("reasoning"),
-                        "model": payload.get("model"),
-                        "tokens": payload.get("tokens"),
-                        "runtime": payload.get("runtime"),
-                        "ts": payload.get("ts"),
-                        "media": payload.get("media"),
-                    }
-                )
+                msg = {
+                    "cursor": int(r["cursor"]),
+                    "message_id": payload.get("message_id"),
+                    "role": role,
+                    "text": payload.get("text", ""),
+                    "reasoning": payload.get("reasoning"),
+                    "model": payload.get("model"),
+                    "tokens": payload.get("tokens"),
+                    "runtime": payload.get("runtime"),
+                    "ts": payload.get("ts"),
+                }
+                # Omit ``media`` when absent (schema: array, not null) — a
+                # ``"media": null`` would break the app's deserialization.
+                if payload.get("media"):
+                    msg["media"] = payload["media"]
+                final.append(msg)
             elif ftype == "message.stop":
+                # Streaming finals carry no media (offers are separate
+                # frames); omit the key (schema: array, not null).
                 final.append(
                     {
                         "cursor": int(r["cursor"]),
@@ -233,7 +237,6 @@ class Outbox:
                         "tokens": payload.get("tokens"),
                         "runtime": payload.get("runtime"),
                         "ts": payload.get("ts"),
-                        "media": None,
                     }
                 )
         # Deduplicate by message_id (keep the latest occurrence), keep order.