Avoid selecting pre-releases for Python downloads without a version request

[zanieb]

Following #7263 the 3.13.0rc2 releases are at the top of the download list but we should not select them unless 3.13 is actually requested.

Prior to this, uv python install would install 3.13.0rc2.

❯ cargo run -- python install --no-config
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/uv python install --no-config`
Searching for Python installations
Installed Python 3.12.6 in 1.33s
 + cpython-3.12.6-macos-aarch64-none

❯ cargo run -- python install --no-config 3.13
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/uv python install --no-config 3.13`
Searching for Python versions matching: Python 3.13
Installed Python 3.13.0rc2 in 1.18s
 + cpython-3.13.0rc2-macos-aarch64-none

[zanieb]

This breaks uv python uninstall --all because it uses PythonDownloadRequest which needs to match here. Tweaking..

[zanieb]

Okay all better

❯ cargo run -- python install --no-config
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/uv python install --no-config`
Searching for Python installations
Installed Python 3.12.6 in 1.20s
 + cpython-3.12.6-macos-aarch64-none
❯ cargo run -- python install --no-config 3.13
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/uv python install --no-config 3.13`
Searching for Python versions matching: Python 3.13
Installed Python 3.13.0rc2 in 1.25s
 + cpython-3.13.0rc2-macos-aarch64-none
❯ cargo run -- python uninstall --all
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/uv python uninstall --all`
Searching for Python installations
Uninstalled 2 versions in 82ms
 - cpython-3.12.6-macos-aarch64-none
 - cpython-3.13.0rc2-macos-aarch64-none
❯ cargo run -- python install --no-config 3.13
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/uv python install --no-config 3.13`
Searching for Python versions matching: Python 3.13
Installed Python 3.13.0rc2 in 1.24s
 + cpython-3.13.0rc2-macos-aarch64-none
❯ cargo run -- python install --no-config
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/uv python install --no-config`
Searching for Python installations
Found: cpython-3.13.0rc2-macos-aarch64-none
Python is already available. Use `uv python install <request>` to install a specific version.

[zanieb]

Cancelled the #7274 release for this — will re-run after merge. Users aren't impacted by this bug since it's unreleased.