Function isRetriableError

Returns true only for transient, retriable failures: request timeouts, network-level connection-closed/reset errors, and "not connected" states.

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

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