atepg: wrap request path transactions in one span

Follow up from #1462.

The store tracer emits one client span per PostgreSQL statement. pgx sends BEGIN and COMMIT as normal statements, so a transaction on the request path shows as separate begin, statement and commit spans next to each other under the RPC span. This is correct but a bit noisy.

Idea from the review: wrap each transaction in one INTERNAL span and nest the statement spans under it. pgx has no hook per transaction, so this needs a small helper around pool.Begin in the store. Three call sites in cmd/ateapi/internal/store/atepg/atepg.go run on the request path. The outbox transactions run in background work without a parent span and produce no spans today.

We keep the begin and commit spans in any case. COMMIT is where the WAL flush time lands, so it carries real latency.