Expose `SystemNode::discover` so callers can locate and validate Node
without also requiring npm or initializing Zed's cache. It accepts a
configured executable or an explicit search PATH, preserves the Node 22
minimum, and reports typed lookup, execution, and version errors with
guidance.

Zed's existing system runtime reuses the validation before its existing
npm setup. Configured overrides and PATH order remain authoritative.
Selected paths are made absolute without resolving symlinks, and
absolute executables remain usable after the process's working directory
is removed. Managed downloads and npm policy are unchanged.

Release Notes:

- N/A