#4190 swamp-uat private-registry tests leave npm/<port> dirs in the shared Deno cache

swamp-uat's private-registry tests (packages named swamp-uat-private-) point Deno's npm resolution at a localhost registry on a random port, using the shared ~/.swamp/deno-cache as DENO_DIR. Deno caches npm metadata per registry port under npm//, and the tests leave those directories behind: 35 on one host on 2026-10-30, dated 2026-10-26 through 2026-10-30, each with a swamp-uat-private-* package and a zod packument listing only 4.6.7.

Impact: any later process that runs a registry on a reused port with the same cache reads the stale packument. That failed swamp's dependency input vectors test (zod-3 vector: Could not resolve 'npm:zod@3.23.0'), tracked as swamp-club#4189 and fixed on the swamp side by clearing the port directory before use. Other consumers of the shared cache stay exposed, and the directories accumulate.

Expected: each swamp-uat test that serves a registry on a port removes DENO_DIR/npm/ (and the remote/http/127.0.0.1_PORT entries) when it ends, or runs with a per-test DENO_DIR (as swamp's extension_push_lockfile_read_only_test does) with esbuild warmed into it.