test(npm): observability handler-wiring acceptance test (S11 Step 22)

Part 2 of 2 of the npm hosted plan — Step 22: observability hooks.

🗂️ Stacked MRs

Split into 2 stacked MRs to keep each within the reviewable-size guardrail. This part targets part 1's branch (dm/npm-hosted-step-22-impl), so its diff is only the acceptance-test slice — retarget to main once part 1 merges. Review and merge bottom-up.

• feat(npm): observability logging and request me... (!849 - merged) • Dzmitry (Dima) Meshcharakou • 19.2
• test(npm): observability handler-wiring accepta... (!850 - merged) • Dzmitry (Dima) Meshcharakou • 19.2 👈

📦 What this part adds

The handler-wiring acceptance (integration) test for the Step 22 observability wiring.

• wire_npm_observability_integration_test.go — stands up the assembled npm dispatcher (buildNpmDispatcher) over an isolated database and drives one request per endpoint, asserting each carries the canonical structured-log fields and records exactly one request observation under the correct (handler, code) label — on both success and error outcomes, plus the committed tarball size on publish and package_version on the download and dist-tag-PUT paths.

Split out from part 1 (!849 (merged)) so the implementation MR stays within the reviewable-size guardrail.

📚 References

• Plan: https://gitlab.com/gitlab-org/ops/artifact-registry/-/blob/main/docs/plans/2026-05-11-npm-hosted.md (Step 22)
• Spec: https://gitlab.com/gitlab-org/ops/artifact-registry/-/blob/main/docs/specs/S11-npm-hosted.md

Related to #140 (closed)