fix(packagist): Throw on transient errors

[zharinov]

Changes

Treat transient failures from the default Packagist registries as external host errors instead of normal lookup misses.

This covers repo.packagist.org and packagist.org for timeout/reset errors, rate limiting, and 5xx responses. This prevents temporary Packagist availability failures from being converted into no-result lookup outcomes.

Related discussion: #43643

Context

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[RahulGautamSingh]

lgtm except the nitpick