Add AK/SK REST authentication path

[chijiajian]

Summary

• add AK/SK request signing support for ZStack REST APIs
• route mapped Query APIs to /zstack/v1 REST paths instead of /zstack/api message APIs
• return REST_MAPPING_NOT_FOUND for unmapped AK/SK APIs
• document AK/SK configuration and add coverage for REST routing behavior

Tests

• git diff --check
• PYTHONPYCACHEPREFIX=/private/tmp/zstack-mcp-pycache python3 -m py_compile src/zstack_mcp/zstack_client.py src/zstack_mcp/server.py tests/test_zstack_client_access_key.py tests/test_auth_refactor.py
• stub self-check for AK/SK REST QueryZone routing and unmapped API blocking

Note: full pytest was not run because this environment lacks uv/pytest/httpx and the system Python is 3.9 while the project requires Python 3.11+.