Update the lockfile during uv check --no-sync

[zsol]

Summary

uv check --no-sync currently skips the project lock operation along with environment synchronization. This means it does not create a missing uv.lock or update a stale one.

This changes --no-sync to control environment synchronization only. uv check now performs the normal project lock operation, uses the resulting lock for workspace metadata, and then skips synchronizing project dependencies into the environment. When an existing environment is incompatible with the project or an explicit Python request, locking discovers a separate compatible interpreter without replacing or synchronizing that environment.

Explicit lock controls continue to apply when combined with --no-sync:

• --locked validates the lockfile without writing it
• --frozen uses the existing lockfile without freshness validation or writes
• --isolated performs dry-run locking and leaves project state unchanged

As a consequence, malformed lockfiles now produce an error instead of being ignored.

[astral-sh-bot]

uv test inventory changes

This PR changes the tests when compared with the latest main baseline.

• Added tests: 9
• Removed tests: 6
• Changed suites: 1

uv::project: +9 / -6

Added:

• uv::project::check::check_no_sync_creates_lock_without_sync
• uv::project::check::check_no_sync_errors_on_invalid_lockfile
• uv::project::check::check_no_sync_frozen_requires_existing_lock
• uv::project::check::check_no_sync_frozen_uses_existing_lock_without_update
• uv::project::check::check_no_sync_isolated_does_not_write_lock_or_sync
• uv::project::check::check_no_sync_locked_rejects_stale_lock_without_update
• uv::project::check::check_no_sync_locked_requires_existing_lock
• uv::project::check::check_no_sync_updates_stale_lock_without_sync
• uv::project::check::check_no_sync_uses_compatible_lock_interpreter

Removed:

• uv::project::check::check_no_sync_ignores_invalid_lockfile
• uv::project::tree::invert_preserves_conflict_marker_attribution
• uv::project::tree::invert_preserves_dependency_group_attribution
• uv::project::tree::invert_preserves_extra_attribution
• uv::project::tree::invert_preserves_marker_attribution
• uv::project::tree::invert_preserves_marker_split_versions

[Gankra]

We should file a followup to make the same change to run, but that might need to land in the upcoming breaking release.

[Gankra]

You actually don't need to have this be an Option or assign None here.

You can indeed still just do:

let _environment lock;
if !no_sync {
  _environment_lock = ...
}

Rust lets you delay-assign an immutable var, and it's fine to only initialize a var in some paths if it's never read. It will handle the conditional dtor tracking for you. It's IMO slightly more idiomatic and clean to not have the None here, but feel free to disagree.

[zanieb]

I don't think it's breaking to lock on --no-sync, but we can discuss that on the other pull.