Support centralised environments in uv venv

[EliteTK]

Summary

With the centralised project environments preview feature enabled, uv venv invoked with no path and UV_PROJECT_ENVIRONMENT unset will now use the environment cache bucket as the backing store for the environment. The key selection logic is the same as for centralised project environments.

There's are some cases of note:

• With the feature enabled:
  • Unless you explicitly pass --no-clear, existing environments will be clobbered without prompting
  • If you pass --allow-existing this will avoid clearing the centralised environment, before re-using it, but will still clobber local environments when creating the link. This might sound weird, but the alternatives are:
    • Disable the feature if .venv exists, but if .venv doesn't exist, instead operate on any "existing" environment in the cache (this gets more confusing the more you think about it)
    • Don't touch .venv if it exists, but otherwise operate on any "existing" environment in the cache
    • Complain if .venv exists but is not referencing the cache (somewhat sane sounding... except also somewhat heavy and still weird)
• With the feature disabled:
  • Whenever we would normally clear an environment, if it's a link to the cache, we just delete the link (we would normally clear the target of the link). I consider this not to be breaking because a .venv that is a link into the cache should never have been a thing prior to the feature.
  • The user is not prompted to remove links to the cache.

See also #18214.

Test Plan

A range of new tests covering a variety of normal and corner cases.

[astral-sh-bot]

uv test inventory changes

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

• Added tests: 5
• Removed tests: 1
• Changed suites: 1

uv::python: +5 / -1

Added:

• uv::python::venv::create_centralized_project_environment
• uv::python::venv::create_centralized_project_environment_bypasses
• uv::python::venv::create_centralized_project_environment_link_failure
• uv::python::venv::create_centralized_project_environment_no_cache
• uv::python::venv::create_centralized_project_environment_with_seed_packages

Removed:

• uv::python::venv::create_venv_centralized_project_envs_warning

[zsol]

does uv cache prune remove orphaned envs after someone switches off the preview flag and recreates their .venv?

[zsol]

this needs an update mentioning the return value

[zanieb]

#20010

[EliteTK]

We have no way to detect an in-use .venv because it could be on a currently unmounted filesystem or another machine. We don't have a project database and symlinks don't increment anything similar to the hardlink count. We have resigned to the fact that we don't have a way to determine what "orphaned" means and we clear all cached environments always, regardless of the feature.