fix(qq): add configurable message format and onboard backfill

tests/test_qq_channel.py

@@ -94,3 +94,32 @@ async def test_send_c2c_message_uses_plain_text_c2c_api_with_msg_seq() -> None:
         "msg_seq": 2,
     }
     assert not channel._client.api.group_calls
+
+
+@pytest.mark.asyncio
+async def test_send_group_message_uses_markdown_when_configured() -> None:
+    channel = QQChannel(
+        QQConfig(app_id="app", secret="secret", allow_from=["*"], msg_format="markdown"),
+        MessageBus(),
+    )
+    channel._client = _FakeClient()
+    channel._chat_type_cache["group123"] = "group"
+
+    await channel.send(
+        OutboundMessage(
+            channel="qq",
+            chat_id="group123",
+            content="**hello**",
+            metadata={"message_id": "msg1"},
+        )
+    )
+
+    assert len(channel._client.api.group_calls) == 1
+    call = channel._client.api.group_calls[0]
+    assert call == {
+        "group_openid": "group123",
+        "msg_type": 2,
+        "markdown": {"content": "**hello**"},
+        "msg_id": "msg1",
+        "msg_seq": 2,
+    }