fix(install_pypi): preserve files when migrating dependency from pypi to conda

[aoright]

Description

When an environment is updated such that a package previously installed via PyPI is moved to conda dependencies, conda installs the conda package into the environment prefix first.

During the subsequent PyPI update phase, InstallPlanner::plan inspects site-packages. Previously, packages not in required_dists were partitioned into duplicates only if dists.len() > 1. If conda did not install a separate .dist-info (or replaced it, or only module files exist), dists.len() == 1, causing the old PyPI package to be classified as extraneous. Then remove_packages invoked uv_installer::uninstall(dist), which read the old PyPI RECORD file and deleted all the files that conda had just extracted into site-packages/.

This PR fixes this by:

1. Collecting conda package names and PyPI purls from pixi_records during planning setup.
2. Passing conda_packages to InstallPlanner.
3. Partitioning unrequired distributions into duplicates when dists.len() > 1 || self.conda_packages.contains(name).
4. Ensuring that remove_duplicate_metadata removes only the stale PyPI .dist-info directory without calling uv_installer::uninstall, leaving the conda-installed package files intact.

Fixes #6902

How Has This Been Tested?

• Added unit test test_conda_package_not_extraneous_when_replacing_pypi in crates/pixi_install_pypi/src/plan/test/mod.rs.
• Ran cargo test -p pixi_install_pypi across unit and integration tests.
• Ran cargo clippy -p pixi_install_pypi --all-targets and cargo fmt --check.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Google Antigravity

Checklist:

• I have performed a self-review of my own code
• I have added sufficient tests to cover my changes.