Write requested python variant to pin file in `uv init` (#12870)
## Summary
Closes #12855
This PR also fixed an issue, where `python_request` was matched against
`PythonVersion::Default`. Previously, if `python_request` was `3.13t`,
it would match the last branch, triggering a download of the Python
version if it wasn't already installed.
https://github.com/astral-sh/uv/blob/6b7f60c1eaa840c2e933a0fb056ab46f99c991a5/crates/uv/src/commands/project/init.rs#L421-L448
```console
❯ uv init -v --managed-python --python 3.13t foo
DEBUG uv 0.6.14 (a4cec56dc 2025-04-09)
DEBUG Searching for Python 3.13t in managed installations
DEBUG Searching for managed installations at `/Users/Jo/.local/share/uv/python`
DEBUG Found managed installation `cpython-3.13.1-macos-aarch64-none`
DEBUG Found `cpython-3.13.1-macos-aarch64-none` at `/Users/Jo/.local/share/uv/python/cpython-3.13.1-macos-aarch64-none/bin/python3.13` (managed installations)
DEBUG Skipping interpreter at `/Users/Jo/.local/share/uv/python/cpython-3.13.1-macos-aarch64-none/bin/python3.13` from managed installations: does not satisfy request `3.13t`
DEBUG Skipping incompatible managed installation `cpython-3.12.8-macos-aarch64-none`
DEBUG Skipping incompatible managed installation `pypy-3.11.11-macos-aarch64-none`
DEBUG Requested Python not found, checking for available download...
DEBUG Acquired lock for `/Users/Jo/.local/share/uv/python`
DEBUG Using request timeout of 30s
INFO Fetching requested Python...
Downloading cpython-3.13.3+freethreaded-macos-aarch64-none (49.9MiB)
DEBUG Downloading https://github.com/astral-sh/python-build-standalone/releases/download/20250409/cpython-3.13.3%2B20250409-aarch64-apple-darwin-freethreaded%2Bpgo%2Blto-full.tar.zst to temporary location: /Users/Jo/.local/share/uv/python/.temp/.tmpfoOLkE
DEBUG Extracting cpython-3.13.3%2B20250409-aarch64-apple-darwin-freethreaded%2Bpgo%2Blto-full.tar.zst
Downloaded cpython-3.13.3+freethreaded-macos-aarch64-none
DEBUG Moving /Users/Jo/.local/share/uv/python/.temp/.tmpfoOLkE/python/install to /Users/Jo/.local/share/uv/python/cpython-3.13.3+freethreaded-macos-aarch64-none
DEBUG Released lock at `/Users/Jo/.local/share/uv/python/.lock`
DEBUG Writing Python versions to `/private/tmp/foo/.python-version`
Initialized project `foo` at `/private/tmp/foo`
❯ cat foo/.python-version
3.13
```
After this PR, uv will not try to download it:
```console
❯ uv python uninstall 3.13t
❯ cargo run -- init -v --managed-python --python 3.13t bar
DEBUG uv 0.6.14+15 (6b7f60c1e 2025-04-12)
DEBUG Writing Python versions to `/private/tmp/bar/.python-version`
Initialized project `bar` at `/private/tmp/bar`
❯ cat bar/.python_version
3.13t
```
