feat: support voice messages in Slack adapter

[chenjian-agent]

Feature Request

Currently the Slack adapter does not handle voice/audio messages sent by users. Voice messages are a native Slack feature (especially on mobile), and ignoring them leads to silent failures where the bot simply does not respond.

Description

When a user sends a voice message in Slack, the bot should:
1. Detect the audio file attachment from the Slack event payload
2. Download or stream the audio content
3. Transcribe it (e.g., via Whisper or a similar STT service) and pass the text to the agent
4. Respond in the same thread/channel as usual

Use Case

- Mobile users frequently send voice messages instead of typing, especially for longer or more complex requests
- Teams using Slack as a primary interface expect the bot to handle all common message types, not just text
- Voice input lowers the barrier for non-technical users to interact with the agent
- Enables hands-free workflows where typing is inconvenient

Related

https://discord.com/channels/1491295327620169908/1491365150664560881/1494665797673423040

[chenjian-agent]

Related Discord discussion: https://discord.com/channels/1491295327620169908/1491365150664560881/1494665797673423040

[thepagent]

Slack adapter should already support voice/image. No?