feat(channels/telegram): Telegram 渠道插件（Bot API 长轮询/Webhook + file_id 媒体 + 主动推送强）

plugins/channels/telegram/errors.go

@@ -0,0 +1,17 @@
+package telegram
+
+import "errors"
+
+// Sentinel errors returned by the Telegram adapter. Consumers test for them
+// with errors.Is; failures that mean "not connected" wrap adapter.ErrNotConnected
+// so the shared channel sentinel still matches.
+var (
+	// ErrMissingToken is returned when no BotToken is configured.
+	ErrMissingToken = errors.New("telegram: missing bot token")
+	// ErrBadResponse is returned when the Bot API replies with OK=false or an
+	// unexpected payload shape.
+	ErrBadResponse = errors.New("telegram: bad bot api response")
+	// ErrNoChatTarget is returned when an outbound message carries no usable
+	// conversation/group identifier.
+	ErrNoChatTarget = errors.New("telegram: message has no target chat")
+)