fix: verify PR release artifacts before creating tags

This PR moves the artifact verification step before tag creation and release
deletion, so we fail early if no artifacts are available rather than creating
side effects that would need to be cleaned up.

Addresses feedback from https://leanprover.zulipchat.com/#narrow/channel/399079-infrastructure/topic/PR.20toolchain.20even.20if.20test.20suite.20fails/near/570482678

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>