Parse and diagnose a resource file. When the saphyr-spans feature is
enabled and the file parses cleanly under saphyr, diagnostics carry
source spans for root-level codes. Otherwise spans are absent.
Parse a resource definition from a YAML string using the default
(serde_yaml) parser. Used by tests and tooling that already have the
YAML in memory. Endpoint defaults are NOT applied; use parse_resource
if you need them.