MCP server's compare_datasets tool has no proxy_hints support at all

[yakew7]

Where: faircode/mcp_server.py's _compare_datasets_impl (around line 106) and its @server.tool() wrapper.

faircode/cli.py's compare subcommand supports --proxy-hints, computing proxy_hints_a/proxy_hints_b separately for both datasets (added in #247, mirroring profile's existing support). The MCP server's profile_dataset tool has a proxy_hints parameter, but compare_datasets has no equivalent parameter or logic whatsoever - an MCP-calling agent can get proxy hints from profiling one dataset, but never from comparing two.

Suggested fix: add a proxy_hints: bool = False parameter to the compare_datasets MCP tool, computing proxy_hints_a/proxy_hints_b the same way cli.py's compare branch already does, and surfacing the same RuntimeError (missing scipy) as a clean tool error the way profile_dataset's proxy_hints parameter already does.