fix: proper handling of upstream registry 404s

[Tilian]

Fixes #208.

[andrew]

Please address the remaining 404 paths:

• Add ErrUpstreamNotFound handling to the URL-based artifact handlers for Debian, RPM, NuGet, PyPI, Composer, CRAN, Julia, Conda, and Conan, with route-level tests. These paths still return 502.
• Bump github.com/git-pkgs/registries to v0.6.3, which includes the circuit-breaker fix from registries#44. Version 0.6.2 counts 404s as failures, so repeated misses eventually become 502s.
• Refactor the shared Gem and Hex error handling so the dupl lint check succeeds.

[Tilian]

Thanks for the super quick review!

I've mapped upstream 404s for all remaining artifact handlers via a shared serveArtifactError helper.
OCI's BLOB_UNKNOWN now returns 404 too. Actual fetch failures return INTERNAL_ERROR instead.

Per-ecosystem fetch-fail log lines are now being normalized to "failed to get artifact". The associated ecosystem can be identified through the INFO-level logging (or the URL).