Retrieve a SearchBatch by ID and return it in SearchBatchResponse format.

The two TIN counts are computed per request rather than stored.
tin_matching_pending_count counts the batch's searches still
inside the IRS TIN-match retry budget, so it falls as verdicts land;
while it is positive the batch's exports are a provisional snapshot
that will be regenerated — exports_generated_at moves forward each
time — so a caller that cached a download should re-read this response
before trusting it.

It reaching zero means nothing is left to retry, NOT that every TIN
was verified. tin_matching_unresolved_count is the companion that
says how many rows FINISHED without a verdict and dropped out of the
retry pool (budget spent, retries disabled for the program, or the
search cancelled). Both come from one scan so they cannot disagree.

The two do not partition the batch's TIN rows: a search that is still
running is in neither, because its TIN match has not been attempted to
a conclusion. On a COMPLETED batch two zeroes therefore do mean
every TIN was verified; on one still processing they mean only that
nothing has been given up on yet.