Function isRetriableError

Returns true only for transient, retriable failures: request timeouts, network-level connection-closed/reset errors, "not connected" states, and transient HTTP gateway errors (502, 503, 504).

Uses an allow-list of known transient error patterns so that unknown or server-level errors (e.g., tool runtime failures) are NOT retried:

• timeout — AbortSignal timeout or custom timeout message
• connection closed / reset / refused — network-level transport failures
• not connected — local "not yet connected" guard error
• socket hang up — Node.js HTTP socket-level disconnection
• gateway error 502/503/504 — transient upstream server errors

Everything else (MCPSessionExpiredError, TypeError, rate-limit errors, unknown errors) returns false so callToolWithRetry surfaces them immediately.