Adds common/mcp_service.py with the McpService dataclass (moved from
main.py), BUILTIN_SERVICES list, _filter_by_env, and discover_and_filter.
MCP_SERVICES env var acts as a whitelist (unset=all, empty=zero).
Adds tests/unit/test_mcp_service.py with 12 cases covering: load order,
broken-import handling (warn vs raise by env), missing/wrong-typed
SERVICE, duplicate name and duplicate mount_path detection, env filter
(unset/empty/subset/order), and unknown-name error.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
