Reify @error PayloadAccessor errors fetcher into a named method

R303 reified every datafetcher onto a named <Type>Fetchers method except one: the @error-type errors field on the Transport.PayloadAccessor arm, which still registers graphql-java’s PropertyDataFetcher.fetching(name) (a runtime reflective property read off the parent payload).

R303’s Validator-mirror section proposed reifying it "via the same recordBackedAccessorRead helper the class-backed path uses," but that helper needs an AccessorResolution.Resolved, and ChildField.ErrorsField / Transport.PayloadAccessor carry no resolved accessor: the read is deliberately left to graphql-java’s runtime reflection. Resolving it at generation time means the classifier (FieldBuilder.liftToErrorsField) must reflect the payload class and produce an AccessorResolution.Resolved for the errors slot, a classifier-model change R303 explicitly excluded ("the classifier model is untouched"). So R303 deferred it here rather than reach past the parse-boundary with emit-time reflection.

Scope when this is picked up: resolve the errors-field accessor at classify time (mirroring the PropertyField / RecordField accessor resolution on a class-backed parent), reify the PayloadAccessor arm in FetcherEmitter.bind to a source-only LightFetcher-wrapped read, then reconcile FetcherEmitter.resolvesViaPropertyDataFetcher and the R268 validateOutcomeChildArmSwitch rule (the PropertyDataFetcher escape the predicate guards no longer exists once reified), and retire DataFetcherKind.PROPERTY_FETCHER.