fix(acp): provide valid request for auto-created sessions

[dvd233]

Summary

• Keep the ACP compatibility path that auto-creates a session when a client skips newSession.
• Construct NewSessionRequest with the configured workspace and an empty MCP server list so the fallback satisfies the ACP schema.
• Add a regression assertion for unknown-session prompts and the configured workspace.

Why

The auto-create path was added for clients such as Zed, but it currently calls NewSessionRequest(cwd=None). The ACP schema requires a string cwd and mcpServers, so every unknown-session prompt raises a validation error before the compatibility path can run.

This PR addresses only the unknown-session fallback from #120. It does not change the provider or tool-kind issues described in the same report.

Validation

• uv run pytest tests/test_acp.py tests/test_session_integration.py -q — 7 passed
• uv run python -m compileall -q mini_agent tests/test_acp.py
• git diff --check
• Constructed NewSessionRequest(cwd=..., mcpServers=[]) with agent-client-protocol==0.12.1
• The full suite was also run on Windows; unrelated existing failures remain for missing local config fixtures, PowerShell/bash semantics, Windows file locks, and proxy setup.

AI assistance: Codex was used for diagnosis and test execution; the submitted diff was reviewed for scope and correctness.

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