fix(weixin): check both ret and errcode on send to avoid silent drops

The iLink API signals failures through either `ret` or `errcode`.
`_poll_once` already checked both, but `_send_text` and `_send_media_file`
only checked `errcode`. When the API returned `ret != 0` with
`errcode == 0`, the send appeared successful but the message was never
delivered, causing the "still losing messages" issue.

- Add `_check_response_error` helper that validates both fields
- Use it in `_send_text` and `_send_media_file`
- Add debug log after successful text send for observability
- Add test for nonzero ret with zero errcode

Refs: previous inbound fix (suppress -> explicit try/except)

nanobot/channels/weixin.py

@@ -526,6 +526,22 @@ class WeixinChannel(BaseChannel):
                 f"WeChat session paused, {remaining_min} min remaining (errcode {ERRCODE_SESSION_EXPIRED})"
             )
 
+    @staticmethod
+    def _check_response_error(data: dict, operation: str) -> None:
+        """Check both ``ret`` and ``errcode`` like the reference TS code.
+
+        The iLink API may signal failure through either field (or both).
+        ``_poll_once`` already checks both; outbound send helpers must do
+        the same to avoid silent drops.
+        """
+        ret = data.get("ret", 0)
+        errcode = data.get("errcode", 0)
+        is_error = (ret is not None and ret != 0) or (errcode is not None and errcode != 0)
+        if is_error:
+            raise RuntimeError(
+                f"WeChat {operation} error (ret={ret}, errcode={errcode}): {data.get('errmsg', '')}"
+            )
+
     async def _poll_once(self) -> None:
         remaining = self._session_pause_remaining_s()
         if remaining > 0:
@@ -1123,11 +1139,8 @@ class WeixinChannel(BaseChannel):
         }
 
         data = await self._api_post("ilink/bot/sendmessage", body)
-        errcode = data.get("errcode", 0)
+        self._check_response_error(data, "send text")
-        if errcode and errcode != 0:
+        self.logger.debug("WeChat text sent to {} (client_id={})", to_user_id, client_id)
-            raise RuntimeError(
-                f"WeChat send text error (code {errcode}): {data.get('errmsg', '')}"
-            )
 
     async def _send_media_file(
         self,
@@ -1273,11 +1286,7 @@ class WeixinChannel(BaseChannel):
         }
 
         data = await self._api_post("ilink/bot/sendmessage", body)
-        errcode = data.get("errcode", 0)
+        self._check_response_error(data, "send media")
-        if errcode and errcode != 0:
-            raise RuntimeError(
-                f"WeChat send media error (code {errcode}): {data.get('errmsg', '')}"
-            )
 
 
 # ---------------------------------------------------------------------------

tests/channels/test_weixin_channel.py

@@ -1252,6 +1252,26 @@ async def test_send_text_succeeds_on_zero_errcode() -> None:
     channel._api_post.assert_awaited_once()
 
 
+@pytest.mark.asyncio
+async def test_send_text_raises_on_nonzero_ret_even_when_errcode_zero() -> None:
+    """_send_text must raise when the API returns ret != 0, even if errcode is 0.
+
+    The iLink API signals failure through either field.  Checking only errcode
+    caused silent message drops (responses generated but never delivered).
+    """
+    channel, _bus = _make_channel()
+    channel._client = object()
+    channel._token = "token"
+    channel._api_post = AsyncMock(
+        return_value={"ret": -100, "errcode": 0, "errmsg": "internal error"}
+    )
+
+    with pytest.raises(RuntimeError, match="WeChat send text error.*ret=-100.*errcode=0"):
+        await channel._send_text("wx-user", "hello", "ctx-ok")
+
+    channel._api_post.assert_awaited_once()
+
+
 # ---------------------------------------------------------------------------
 # Tests for _poll_once not silently dropping messages on processing errors
 # ---------------------------------------------------------------------------