refactor(channels): standardize Send error classification with sentinel types

All 12 channel Send methods now return proper sentinel errors (ErrNotRunning,
ErrTemporary, ErrRateLimit, ErrSendFailed) instead of plain fmt.Errorf strings,
enabling Manager's sendWithRetry classification logic to actually work.

- Add ClassifySendError/ClassifyNetError helpers in errutil.go for HTTP-based channels
- LINE/WeCom Bot/WeCom App: use ClassifySendError for HTTP status-based classification
- SDK channels (Telegram/Discord/Slack/QQ/DingTalk/Feishu): wrap errors as ErrTemporary
- WebSocket channels (OneBot/WhatsApp/MaixCam): wrap write errors as ErrTemporary
- WhatsApp: add missing IsRunning() check in Send
- WhatsApp/OneBot/MaixCam: add ctx.Done() check before entering write path
- Telegram Stop: clean up placeholders sync.Map to prevent state leaks

pkg/channels/feishu/feishu_64.go

@@ -91,7 +91,7 @@ func (c *FeishuChannel) Stop(ctx context.Context) error {
 
 func (c *FeishuChannel) Send(ctx context.Context, msg bus.OutboundMessage) error {
 	if !c.IsRunning() {
-		return fmt.Errorf("feishu channel not running")
+		return channels.ErrNotRunning
 	}
 
 	if msg.ChatID == "" {
@@ -115,11 +115,11 @@ func (c *FeishuChannel) Send(ctx context.Context, msg bus.OutboundMessage) error
 
 	resp, err := c.client.Im.V1.Message.Create(ctx, req)
 	if err != nil {
-		return fmt.Errorf("failed to send feishu message: %w", err)
+		return fmt.Errorf("feishu send: %w", channels.ErrTemporary)
 	}
 
 	if !resp.Success() {
-		return fmt.Errorf("feishu api error: code=%d msg=%s", resp.Code, resp.Msg)
+		return fmt.Errorf("feishu api error (code=%d msg=%s): %w", resp.Code, resp.Msg, channels.ErrTemporary)
 	}
 
 	logger.DebugCF("feishu", "Feishu message sent", map[string]any{