test: add ACP integration test

[codefromthecrypt]

Summary

Adds an integration test for the ACP (Agent Client Protocol) server mode. The test verifies the full request/response cycle:

• Protocol initialization handshake
• Session creation
• Prompt submission
• Streaming response chunk reception

This provides regression coverage for ACP functionality and serves as executable documentation for how ACP clients should interact with goose.

Type of Change

• [x] Tests

AI Assistance

• [x] This PR was created or reviewed with AI assistance

Testing

cargo test -p goose --test acp_integration_test

The test spawns goose-cli acp, connects via stdio, and validates the streamed response matches expected content from a mocked OpenAI endpoint (wiremock).

Related Issues

once merged, I can write the impl and test for #6111