Discover and respect .python-version files in parent directories (#6370)

Uses #6369 for test coverage.

Updates version file discovery to search up into parent directories.
Also refactors Python request determination to avoid duplicating the
user request / version file / workspace lookup logic in every command
(this supersedes the work started in
https://github.com/astral-sh/uv/pull/6372).

There is a bit of remaining work here, mostly around documentation.
There are some edge-cases where we don't use the refactored request
utility, like `uv build` — I'm not sure how I'm going to handle that yet
as it needs a separate root directory.

crates/uv-python/src/lib.rs

@@ -17,6 +17,7 @@ pub use crate::prefix::Prefix;
 pub use crate::python_version::PythonVersion;
 pub use crate::target::Target;
 pub use crate::version_files::{
+    DiscoveryOptions as VersionFileDiscoveryOptions, FilePreference as VersionFilePreference,
     PythonVersionFile, PYTHON_VERSIONS_FILENAME, PYTHON_VERSION_FILENAME,
 };
 pub use crate::virtualenv::{Error as VirtualEnvError, PyVenvConfiguration, VirtualEnvironment};