#1026 tools.mcp.call cannot import the MCP client: mcp 2.0.0 has streamable_http_client, code imports streamablehttp_client

Found by the #1001 fork (live, tenant test, runs 68fb8c37 and fca7638c failed with ImportError). uv.lock pins mcp 2.0.0, which exposes streamable_http_client; engine/tools/mcp_client/__init__.py imports streamablehttp_client, so every tools.mcp.call / mcp_list_tools / discovery fails. EARS: When tools.mcp.call runs, the client shall import from the pinned mcp package and complete a call against a real MCP server; a unit test shall import the real symbol (not a fake).