Returns the fully-$defs-resolved input/output JSON Schema for
operation_id under api_version, if known — the same asset
validate_input/validate_output validate against, exposed directly so
services::api_client can read declared parameter/column names and
x-sql-type values off it to build the actual T-SQL call, rather than
re-deriving that from EndpointRecord.input_schema’s unresolved
$refs (see data::store::ENDPOINT_COLUMNS — that column stores the
spec’s requestBody as written, $ref and all).
Returns Err if data doesn’t satisfy operation_id’s output schema
— surfaces upstream API drift as a structured error rather than
silently returning a mismatched response (architecture.md’s call
pipeline). Validates against the given api_version’s schema.