ACP providers return "Invalid message format" error for all models

Description:

When using ACP-based providers (Claude Code ACP and Gemini CLI ACP), all models return "Generation error: Invalid message format" with 0 tokens consumed.

Steps to reproduce:

1. Configure Claude Code (ACP) or Gemini CLI (ACP) provider

2. Select any model (e.g., Gemini 3.1 Pro Preview, Claude Opus)

3. Send any message

4. Error: "Generation error: Invalid message format", Tokens: 0, Tools: 0

Expected behavior: Normal response from the model.

Environment:

• Alma version: 0.0.748

• macOS on Apple Silicon

• claude-agent-acp and gemini CLI both work correctly when used directly in terminal

• MCP servers disabled — same error

• Non-ACP providers (e.g., Google Gemini API directly) work fine

Notes:

• Both Gemini CLI (ACP) and Claude Code (ACP) have the same error, suggesting the issue is in Alma's ACP message serialization, not specific to any backend.

• The CLI processes start successfully (claude-agent-acp --acp --experimental-acp launches and waits for input without errors).