Support audio input

[fjord-oai]

Summary

Adds audio as a structured tool-output content item, gated by model input modalities.

• Adds InputModality::Audio and input_audio function-call output items.
• Converts MCP audio content blocks to Responses-compatible structured output for audio-capable models.
• Fails MCP/code-mode audio output clearly when the selected model does not advertise audio input.
• Adds a strict code-mode audio(...) helper.
• Regenerates app-server protocol schemas so clients can round-trip the new content item shape.