Complete the PyPI refactor started with install: resolution now also
runs through the CommandDispatcher, so both halves of the PyPI story
(solve + install) are reachable outside the workspace code, e.g. for
pixi global.

- Move the resolve pipeline (resolve_pypi, LazyBuildDispatch,
  CondaResolverProvider) from pixi_core::lock_file::resolve into
  pixi_install_pypi::resolve.
- Decouple it from the workspace with a new CondaPrefixProvider trait:
  when uv must build an sdist to get metadata, the provider supplies a
  conda prefix (python interpreter + activation env vars) on demand.
  pixi_core implements it as WorkspaceCondaPrefixProvider wrapping the
  memoized CondaPrefixUpdater and environment activation; both the
  lock-file update path and the satisfiability metadata checks use it.
- Move PypiPackageIdentifier into pixi_install_pypi; the
  satisfiability-specific satisfies() check stays in pixi_core as an
  extension trait.
- Add SolvePypiEnvironmentSpec and
  CommandDispatcher::solve_pypi_environment, deriving the link mode
  from the dispatcher's configured link options.
- Drop the dead conda_task plumbing: LazyBuildDispatch::conda_task was
  never assigned, so resolve_pypi now returns just the locked records
  and the PypiGroupSolved task keeps forwarding None.
- Remove the uv-* dependencies pixi_core no longer needs.

rattler_digest and uv-preview were only used by the PyPI resolve code
that moved to pixi_install_pypi.

Global environments can now declare PyPI packages in the manifest:

    [envs.jupyter]
    channels = ["conda-forge"]
    dependencies = { python = "3.13.*" }
    pypi-dependencies = { jupyterlab = "*" }

After the conda packages are installed, the declared PyPI packages are
resolved with CommandDispatcher::solve_pypi_environment and installed
into the prefix's site-packages with install_pypi_environment — the
first consumer of the dispatcher PyPI pipeline outside the workspace.
The CondaPrefixProvider implementation simply hands out the freshly
installed prefix when a source distribution must be built during
resolution.

Sync detection treats an environment as out of sync when a declared
PyPI package has no dist-info in site-packages (name presence only;
spec changes for an installed package are reconciled on the next
reinstall), and update operations always re-resolve environments with
PyPI dependencies.

PyPI dependencies of global environments can now be managed from the
command line instead of only by editing the manifest:

- pixi global install python --pypi httpx --pypi "flask>=2"
- pixi global add --environment env --pypi httpx
- pixi global remove --environment env httpx (falls back to the PyPI
  dependency when the name is not a conda dependency)

The new Manifest::add_pypi_dependency/remove_pypi_dependency methods
edit the [envs.*.pypi-dependencies] table. Removing the last PyPI
dependency now also cleans pixi-installed packages out of the prefix's
site-packages: leftover detection (dist-info INSTALLER == uv-pixi)
marks the environment out of sync and the installer removes the
packages on the next sync.

PyPI-installed executables are now treated like conda executables when
exposing binaries from a global environment:

- Executables are enumerated from each pixi-installed distribution's
  dist-info RECORD (entries that land in the prefix's binary folders).
  Declared pypi-dependencies are auto-exposed like direct conda
  dependencies; executables of transitive distributions are not
  auto-exposed but are protected from stale-mapping pruning.
- pixi global remove unexposes the removed package's scripts (using a
  snapshot taken before removal) and forces a reinstall: a removed PyPI
  dependency cannot be detected by the presence-based sync check since
  transitive distributions are legitimately undeclared, so the
  installer is invoked explicitly to drop the now-extraneous
  distribution.

The macOS pytest job of the previous run was cancelled by runner
infrastructure.

The PyPI executable enumeration added an extra find_installed_packages
call (a full parse of every conda-meta record) to
executables_of_all_dependencies and ran another one per expose sync in
executables_of_pypi_dependencies, roughly doubling the runtime of the
global integration test suite and pushing the macOS pytest job past
its 10 minute timeout.

Reuse the prefix records that executables_of_all_dependencies already
loads, and skip the prefix entirely in
executables_of_pypi_dependencies when the environment declares no PyPI
dependencies.