feat(gateway): support inbound media and file attachments

[iamninihuang]

What problem does this solve?

Currently, the Custom Gateway adapter only supports receiving plain text (GwContent.text). If a user sends an image, file, or voice message via a custom gateway (like LINE, Telegram, etc.), the bot ignores it because the schema lacks attachment support.

How does it solve it?

This PR introduces comprehensive media support for the Gateway adapter, mirroring the logic already present in Discord and Slack adapters:

1. Schema Update: Expands GwContent with an optional attachments: Vec<GwAttachment> array.
2. Image Processing: Automatically downloads, compresses, and base64 encodes image attachments using media::download_and_encode_image.
3. Text File Inlining: Supports inline reading of text-based files (with a 1MB total size limit and a 5 file cap) via media::download_and_read_text_file.
4. Voice Messages (STT): If stt.enabled is true, it transcribes audio attachments using media::download_and_transcribe. To support this, stt_config is now propagated to the Gateway params.
5. Fallback: Logs an info message and passes a text block for unsupported file types (like PDFs) so the agent knows a file was uploaded.

Is this the best approach?

Yes. It leverages the existing, battle-tested functions in src/media.rs that the Discord and Slack adapters rely on, ensuring consistent memory bounds (e.g., resizing images to 1200px max, 10MB limits, 1MB text caps) and consistent ContentBlock structures across all platforms.

[github-actions]

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[thepagent]

Close in favor of #652
Demo https://discord.com/channels/1491295327620169908/1491365150664560881/1499490944045547562