fix(artifactcache): return after storage.Write error on upload

[baiyuxi930826]

Summary

Fixes #6131: when storage.Write fails during artifact cache upload, the handler now returns after the 500 response instead of continuing into useCache and a second 200 responseJSON (which produced a double body and superfluous response.WriteHeader).

Test plan

• go test ./pkg/artifactcache/ -run TestHandler
• New regression case: force Write failure (storage root is a file) and assert single 500 JSON error body

[baiyuxi930826]

Friendly ping for review when you have a moment.

This is a small fail-closed fix for #6131: return after storage.Write failure on artifact cache upload so we don't double-write a 200 after a 500. Includes a regression test that forces Write failure (storage root is a file) and asserts a single 500 JSON body.

Local: go test ./pkg/artifactcache/ -run TestHandler passes on this branch.

[louzt]

Verified locally against act master HEAD 4f41128 — the missing return after responseJSON(w, r, 500, err) correctly prevents the second WriteHeader(200) call. The new regression test upload write failure returns 500 once (bad rootDir path) is a clean way to assert the single-response contract. Happy to retest if you rebase or want a second reviewer.