Retire the always-empty ErrorChannel.Mapped slots on the polymorphic and table-interface @service variants

Four root @service field variants carry an Optional<ErrorChannel.Mapped> component that is structurally always empty, and the two sides disagree about whether that dead slot makes the field a channel carrier. FieldBuilder.buildServiceField resolves the channel through resolveServiceOutcomeChannel, which returns NoChannel for anything but a ResultReturnType; the polymorphic and table-interface arms are built from ServiceDirectiveResolver.Resolved.Polymorphic (a PolymorphicReturnType) and .TableBound (a TableBoundReturnType), so MutationServicePolymorphicField, MutationServiceTableInterfaceField, QueryServicePolymorphicField, and QueryServiceTableInterfaceField can never hold a present channel. The Query-side pair nonetheless implements WithErrorChannel while the Mutation-side pair implements only ServiceField, so a channel-agnostic reader consuming instanceof WithErrorChannel sees two of the four for no stated reason.

The dead slot has already been paid for twice. MappingsConstantNameDedup.withResolvedChannel claims to pattern-match every WithErrorChannel variant and omits both Query-side arms, landing them on its default -> throw new IllegalStateException; that throw is unreachable only because the slot is always empty, i.e. an invariant the type does not carry is what keeps the switch honest. CatalogBuilder.errorChannelName likewise folds over a slot that has one value.

Decide the shape once: either these returns genuinely cannot host an @error payload, in which case drop the component and the WithErrorChannel implementation from all four and let the dedup switch become exhaustive over what remains; or the omission is a gap, in which case resolveServiceOutcomeChannel should admit them and the dedup switch needs the two missing arms. The second reading overlaps R397, which wants more query return shapes to host @error; if that item lands first, this one is the audit of what its widening leaves behind. Either way the pipeline-tier fixtures that currently synthesise a channel on a root @service field need to state which variants are legitimate hosts.