fix(artifactcache): return after storage.Write error on upload (rebase of #6132)

[louzt]

Summary

Fix silent data loss in artifact upload: when storage.Write fails mid-upload, the handler currently continues processing instead of returning the error.

What

• Return early after storage.Write error in artifact upload path
• Test verifies error propagation through handler response

Rebase status

Rebased against current master (4f41128). Zero conflicts. Tests pass.

[mergify]

⚠️ The sha of the head commit of this PR conflicts with #6132. Mergify cannot evaluate rules on this PR. Once #6132 is merged or closed, Mergify will resume processing this PR. ⚠️

[louzt]

Closing this PR — I verified the missing-return fix locally against act master HEAD 4f41128; the head commit is the same SHA as the still-active #6132 from @baiyuxi930826, so no added value in opening a duplicate. Thanks to the original author for the fix.