feat(channel): add WeCom channel support

**Background**

The channel system in Agent Service currently supports Feishu and Discord. WeCom
(企业微信) is one of the most widely used enterprise IM platforms, and adding WeCom
channel support will significantly broaden the applicability of Agent Service in
enterprise scenarios.

**Changes**
- Add a new `WeComChannel` class under `agentscope.app.channel` module (inherited from
`ChannelBase`), considering to support (depends on the application SDK side):
- Multimodal file transfer
- List conversations and users on the WeCom side
- Send files and messages to specified users or groups
- Access the corresponding knowledge base
- Update docs in https://github.com/agentscope/docs

Start in the `agentscope.app.channel` module and inspect `ChannelBase` plus existing Feishu/Discord channel implementations, since the issue explicitly says the new class belongs there. Check how channels are wired and exposed in that package before adding `WeComChannel` with conversation/user listing, messaging/files, and KB access behavior. Then update documentation in the linked `agentscope/docs` location to include WeCom support and usage notes. Completion is when a `WeComChannel` class exists in-module and docs clearly describe its capabilities.

Escrita pelo modelo de indexação a partir do texto da issue.