feat(channel): add Telegram channel support

**Background**

The channel system in Agent Service currently supports Feishu and Discord. Telegram is a widely adopted messaging platform with a well-established bot API, making it a natural fit for agent-based interactions. Adding Telegram channel support will extend the reach of Agent Service to a large international user base.

**Changes**

- Add a new `TelegramChannel` class under `agentscope.app.channel` module (inherited from `ChannelBase`), considering to support (depends on the Telegram Bot API side):
- `ListChats`: list all available chats accessible by the bot
- `ListChatMembers`: list all members in a specified chat
- `SendMessage`: send a text message to a specified chat or user
- `SendFile`: send a file attachment to a specified chat or user
- `SendImage`: send an image to a specified chat or user
- More tools to be considered based on the capabilities provided by the Telegram Bot API

Begin with `agentscope.app.channel` and `ChannelBase` to confirm how channel adapters are defined, then check the existing Feishu/Discord channel files for method patterns. Add a new `TelegramChannel` class in the same area with the requested tool methods and wire it into channel registration/exports used by the package entry point. Run the relevant channel tests or test entry and mark complete when Telegram appears as a supported channel and those methods are available through normal channel lookup.

Rédigé par le modèle d'indexation à partir du texte de l'issue.