fix(channels): prevent retry amplification and silent message loss across channels

Audited all channel implementations for overly broad exception handling
that causes retry amplification or silent message loss during network
errors. This is the same class of bug as #3050 (Telegram _send_text).

Fixes by channel:

Telegram (send_delta):
- _stream_end path used except Exception for HTML edit fallback
- Network errors (TimedOut, NetworkError) triggered redundant plain
  text edit, doubling connection demand during pool exhaustion
- Changed to except BadRequest, matching the _send_text fix

Discord:
- send() caught all exceptions without re-raising
- ChannelManager._send_with_retry() saw successful return, never retried
- Messages silently dropped on any send failure
- Added raise after error logging

DingTalk:
- _send_batch_message() returned False on all exceptions including
  network errors — no retry, fallback text sent unnecessarily
- _read_media_bytes() and _upload_media() swallowed transport errors,
  causing _send_media_ref() to cascade through doomed fallback attempts
- Added except httpx.TransportError handlers that re-raise immediately

WeChat:
- Media send failure triggered text fallback even for network errors
- During network issues: 3×(media + text) = 6 API calls per message
- Added specific catches: TimeoutException/TransportError re-raise,
  5xx HTTPStatusError re-raises, 4xx falls back to text

QQ:
- _send_media() returned False on all exceptions
- Network errors triggered fallback text instead of retry
- Added except (aiohttp.ClientError, OSError) that re-raises

Tests: 331 passed (283 existing + 48 new across 5 channel test files)

Fixes: #3054
Related: #3050, #3053

tests/channels/test_telegram_channel.py

@@ -387,6 +387,84 @@ async def test_send_delta_stream_end_treats_not_modified_as_success() -> None:
     assert "123" not in channel._stream_bufs
 
 
+@pytest.mark.asyncio
+async def test_send_delta_stream_end_does_not_fallback_on_network_timeout() -> None:
+    """TimedOut during HTML edit should propagate, never fall back to plain text."""
+    from telegram.error import TimedOut
+
+    channel = TelegramChannel(
+        TelegramConfig(enabled=True, token="123:abc", allow_from=["*"]),
+        MessageBus(),
+    )
+    channel._app = _FakeApp(lambda: None)
+    # _call_with_retry retries TimedOut up to 3 times, so the mock will be called
+    # multiple times – but all calls must be with parse_mode="HTML" (no plain fallback).
+    channel._app.bot.edit_message_text = AsyncMock(side_effect=TimedOut("network timeout"))
+    channel._stream_bufs["123"] = _StreamBuf(text="hello", message_id=7, last_edit=0.0)
+
+    with pytest.raises(TimedOut, match="network timeout"):
+        await channel.send_delta("123", "", {"_stream_end": True})
+
+    # Every call to edit_message_text must have used parse_mode="HTML" —
+    # no plain-text fallback call should have been made.
+    for call in channel._app.bot.edit_message_text.call_args_list:
+        assert call.kwargs.get("parse_mode") == "HTML"
+    # Buffer should still be present (not cleaned up on error)
+    assert "123" in channel._stream_bufs
+
+
+@pytest.mark.asyncio
+async def test_send_delta_stream_end_does_not_fallback_on_network_error() -> None:
+    """NetworkError during HTML edit should propagate, never fall back to plain text."""
+    from telegram.error import NetworkError
+
+    channel = TelegramChannel(
+        TelegramConfig(enabled=True, token="123:abc", allow_from=["*"]),
+        MessageBus(),
+    )
+    channel._app = _FakeApp(lambda: None)
+    channel._app.bot.edit_message_text = AsyncMock(side_effect=NetworkError("connection reset"))
+    channel._stream_bufs["123"] = _StreamBuf(text="hello", message_id=7, last_edit=0.0)
+
+    with pytest.raises(NetworkError, match="connection reset"):
+        await channel.send_delta("123", "", {"_stream_end": True})
+
+    # Every call to edit_message_text must have used parse_mode="HTML" —
+    # no plain-text fallback call should have been made.
+    for call in channel._app.bot.edit_message_text.call_args_list:
+        assert call.kwargs.get("parse_mode") == "HTML"
+    # Buffer should still be present (not cleaned up on error)
+    assert "123" in channel._stream_bufs
+
+
+@pytest.mark.asyncio
+async def test_send_delta_stream_end_falls_back_on_bad_request() -> None:
+    """BadRequest (HTML parse error) should still trigger plain-text fallback."""
+    from telegram.error import BadRequest
+
+    channel = TelegramChannel(
+        TelegramConfig(enabled=True, token="123:abc", allow_from=["*"]),
+        MessageBus(),
+    )
+    channel._app = _FakeApp(lambda: None)
+
+    # First call (HTML) raises BadRequest, second call (plain) succeeds
+    channel._app.bot.edit_message_text = AsyncMock(
+        side_effect=[BadRequest("Can't parse entities"), None]
+    )
+    channel._stream_bufs["123"] = _StreamBuf(text="hello <bad>", message_id=7, last_edit=0.0)
+
+    await channel.send_delta("123", "", {"_stream_end": True})
+
+    # edit_message_text should have been called twice: once for HTML, once for plain fallback
+    assert channel._app.bot.edit_message_text.call_count == 2
+    # Second call should not use parse_mode="HTML"
+    second_call_kwargs = channel._app.bot.edit_message_text.call_args_list[1].kwargs
+    assert "parse_mode" not in second_call_kwargs or second_call_kwargs.get("parse_mode") is None
+    # Buffer should be cleaned up on success
+    assert "123" not in channel._stream_bufs
+
+
 @pytest.mark.asyncio
 async def test_send_delta_stream_end_splits_oversized_reply() -> None:
     """Final streamed reply exceeding Telegram limit is split into chunks."""