fix: normalize attachments to core Attachment type in message handler

Upstream AgentApi responses can return attachment objects that don't
implement the Attachment dataclass. _to_core_attachments coerces them
via duck-typing so OutgoingMessage always carries typed Attachment
instances regardless of the upstream response shape.

core/handlers/message.py

@@ -1,7 +1,35 @@
 # core/handlers/message.py
 from __future__ import annotations
 
-from core.protocol import IncomingMessage, OutgoingMessage, OutgoingTyping
+from core.protocol import Attachment, IncomingMessage, OutgoingMessage, OutgoingTyping
+
+
+def _infer_attachment_type(mime_type: str | None) -> str:
+    if not mime_type:
+        return "document"
+    if mime_type.startswith("image/"):
+        return "image"
+    if mime_type.startswith("audio/"):
+        return "audio"
+    if mime_type.startswith("video/"):
+        return "video"
+    return "document"
+
+
+def _to_core_attachments(raw: list) -> list[Attachment]:
+    result = []
+    for a in raw:
+        if isinstance(a, Attachment):
+            result.append(a)
+        else:
+            result.append(Attachment(
+                type=getattr(a, "type", None) or _infer_attachment_type(getattr(a, "mime_type", None)),
+                url=getattr(a, "url", None),
+                filename=getattr(a, "filename", None),
+                mime_type=getattr(a, "mime_type", None),
+                workspace_path=getattr(a, "workspace_path", None),
+            ))
+    return result
 
 
 def _start_command(platform: str) -> str:
@@ -38,6 +66,6 @@ async def handle_message(event: IncomingMessage, auth_mgr, platform, chat_mgr, s
             chat_id=event.chat_id,
             text=response.response,
             parse_mode="markdown",
-            attachments=list(getattr(response, "attachments", [])),
+            attachments=_to_core_attachments(getattr(response, "attachments", [])),
         ),
     ]