[feature] Use a read-only MCP tool as a connection health check #2354

The problem

The MCP health check connects and lists tools. That passes even when the account behind the connection can't run tools, for example when the upstream session expired or lacks a scope. A tool call can also fail with isError: true over HTTP 200 while discovery still looks healthy.

#2300 reports a successful health check followed by Notion tool calls returning 401. A suitable tool check could detect that failure; fixing its authentication cause is separate work.

Proposal

Let the user pick one read-only MCP tool and its JSON arguments as the connection's health check. The check calls only that tool and fails on an error result. Executor should not pick a tool automatically.

Without a configured tool, keep the current connect-and-list check.