Parses OpenAPI 3.x documents into HandlerRegistration bundles with
reqwest-backed forwarding handlers that inject credentials from
OperationContext.capabilities (no-env-vars invariant, ADR-014).
Error codes are prefixed HTTP_<status> (ADR-023); ops are Internal
leaves with FromOpenAPI provenance (ADR-015/022). SSE subscriptions
are consumed via parseSSEFrames; JSON/text/binary response branching
mirrors the TS prior art.