Add replaceFilename helper for artifact renames

[Copilot]

Artifact tests failed because a replaceFilename helper was referenced but missing, leaving filename renames unable to preserve original extensions.

• Filename utility: Implemented replaceFilename to keep the original file extension when renaming artifacts and exported it via Artifact.
• Behavior coverage: Existing unit tests for extension preservation and no-extension cases now pass.

Example:

replaceFilename("original.png", "new.txt") // "new.png"
replaceFilename("filename", "new.txt")     // "new"

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• registry.npmmirror.com
  • Triggering command: /usr/local/bin/node node /usr/local/bin/pnpm install (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

review this project, commit issues if you found any problems

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