Add API command coverage tests

[jixianyihao]

Summary

• Add an in-process ACP dispatch test that exercises all registered top-level agent request and notification handlers through the real ACP client connection.
• Add focused tests for advertised slash commands, session config options, mode/config mutations, cancellation, replayed history, logout, and supported prompt content block mapping.
• Keep the production ACP registration behavior intact by extracting the handler registration chain behind a small internal AgentApi abstraction for testability.

Validation

• cargo fmt --check
• cargo test

[jixianyihao]

Hi @benbrandt, could you take a look when you have a chance? This PR adds focused ACP API and slash-command coverage tests, with the new tests split into dedicated test modules.

[benbrandt]

Hi thanks for taking the time to dig in!
Development is moving to agentclientprotocol/codex-acp. The new adapter is built on the new Codex App Server, and we are pooling implementation and maintenance work across teams there. If this is still needed, please open a PR for that one. Thanks!