Extract the shared npm remote integration fixture harness (three rtInt*/rp* copies)

Problem

internal/format/npm compiles three near-identical remote fixture harnesses into one integration test binary under -tags=integration. The newest, rtInt* in internal/format/npm/remote_tarball_integration_test.go, is roughly 70 lines that restate rp* equivalents already in the binary from internal/format/npm/remote_packument_harness_integration_test.go:

• rtIntNewFixture against rpNewFixture,
• rtIntSHA256, rtIntETag and count against their rp* twins,
• rtIntRegisterCacheCleanup, including the whole DELETE FROM npm_remote_* sweep.

The tarball suite already imports rpTxRunner from that file, so the two are not separated by a package boundary. Nothing forced the copy; the copy was cheaper than the extraction at the time.

Why it is worth closing

Three copies of one cleanup sweep is three places a table added to the npm remote cache schema has to be remembered. A copy that is missed leaves rows behind, and the symptom is a neighbouring test failing later in the run on state it did not create, which reads as flakiness rather than as a missing DELETE.

What the extraction needs

The clean version widens rpSeedRemoteBinding so one seeder can express the tarball suite's explicit cache_validity_hours alongside the packument suite's column defaults. That helper lives in the packument harness file, which belongs to a different MR in the same stack, so the change was deferred rather than made inside !1821 (merged).

Once the stack has landed, the extraction is a single test-only change: one shared fixture builder, one cleanup sweep, and the rtInt* and rp* wrappers reduced to whatever each suite genuinely configures differently.

Source

Raised in review of !1821 (merged), where the reviewer called the cross-MR edit reasonable to defer and asked for a follow-up.