[Artifact cache] Validate local failure handling and build fallback #9524

Goal

Ensure cache failures cannot cause incomplete artifacts to be consumed or incorrectly suppress required build work.

Work

Extend existing integrity tests to exercise the complete consumer-build path with:

• Missing archives or inventories.
• Digest mismatches and incomplete bundles.
• Invalid archive members or unsafe paths.
• Unavailable cache metadata.
• Interrupted restoration and publication.
• A bad newest production with an older eligible production available.

Define which failures trigger normal-build fallback and which remain operation errors.

Configuration and build failures must remain visible failures.

Acceptance criteria

• Corrupt or incomplete productions are rejected.
• An older valid candidate is tried where supported.
• Without a usable candidate, required build work executes.
• Failed restoration leaves no partial stage enabled as prebuilt.
• Failed publication creates no ready entry for incomplete outputs.
• Configuration and build failures are not hidden by cache fallback.
• Diagnostics explain the rejection, retry or fallback.

Dependencies

#9522 Existing isolated integrity tests can be extended beforehand.