Document ACP extension methods and fix available commands ordering

[wangtsiao]

• Document the current ACP core methods, server-to-client ACP updates/requests, and Devo _devo/* extension methods.
• Move ACP available commands advertisement into a session state snapshot path.
• Send the available commands snapshot after session/new, session/load, and session/resume responses instead of inline before the response.
• Add coverage for available_commands_update arriving after the session response and containing compact, goal, and research.