A server that doesn't implement the optional 'ping' utility answers a
keepalive ping with JSON-RPC method-not-found. _is_method_not_found_error
latches that condition so the probe falls back to list_tools instead of
reconnect-looping.
The substring fallback only matched 'method not found' / '-32601' /
'not found: ping'. Servers that surface method-not-found as the common
'Unknown method: <name>' phrasing without a structural -32601 code (e.g.
agentmemory's MCP server) slipped through, so the fallback never latched
and the keepalive reconnect-looped every cycle.
Add 'unknown method' to the substring fallback so the ping->list_tools
keepalive fallback latches for these servers too.
Fixes#50028.
472c068159