uv-lock hook can pass with an outdated lockfile when UV_FROZEN=1

With UV_FROZEN=1, the uv-lock hook can pass even when the lockfile is outdated.

This caused problems for us in CI, where UV_FROZEN=1 is commonly set: a change that should have required a lockfile update was not caught.

Expected: Hook fails when lockfile is outdated.
Actual: Hook does not update the lockfile and does not fail, so lockfile drift can pass unnoticed.
Workaround: Set UV_FROZEN=0 for uv-lock hook execution only.

Start by tracing the uv-lock hook entry point and how it handles UV_FROZEN=1. Reproduce the issue with an outdated lockfile, then verify that the hook fails rather than passing; add or update the relevant test if the repository provides one.

Written by the indexing model from the issue text.