When an MCP server configured as streamableHttp or SSE is unreachable,
streamable_http_client's anyio task group cleanup raises RuntimeError /
ExceptionGroup that escapes the caller's try/except and crashes the
event loop with "Unhandled exception in event loop".
Fix: add a lightweight TCP probe (_probe_http_url) before entering the
MCP SDK transport. If the port is closed, the server is skipped with a
warning instead of crashing. stdio transport is not probed (local
process).
Closes#3739
