NCBI's eLink (and other endpoints) occasionally drops the root element
when the upstream backend connection fails mid-response, yielding just
`<?xml ?>` + DOCTYPE with no body. The XMLValidator was rejecting these
as malformed and the framework classified them as SerializationError
(non-retryable), so the retry chain short-circuited on what is in fact
a transient outage. response-handler now reclassifies empty bodies as
serviceUnavailable({ reason: 'ncbi_unreachable' }) so the retry/backoff
path absorbs the failure. Clears intermittent pubmed_find_related errors.

Also drops the unreachable `if (firstResult?.ERROR)` branch in
pubmed_find_related — response-handler throws on `<ERROR>` payloads
upstream of the tool handler, so the branch was dead.

539 tests pass; bun run devcheck clean.