feat: Increase default stdio buffer limit and allow configuration

[jacoblee93]

Summary

The default stdio stream buffer size is only 64kb, which means you get errors like this when trying to send messages larger than that via ACP:

Filesizes above that are quite common for multimodal data

The new default of 50MB should handle most practical multimodal cases. Definitely open to tweaking this default though if anyone has strong feelings! And it's now configurable if needed

Related issues

Testing

Added tests and ran appropriate make commands

Docs & screenshots

Not sure how to best document, assuming this will automatically propagate to API refs?

Checklist

• Conventional Commit title (e.g. feat:, fix:).
• Tests cover the change or are not required (explain above).
• Docs/examples updated when behaviour is user-facing.
• Schema regenerations (make gen-all) are called out if applicable.