Respect .python-version files and fetch manged toolchains in uv project commands (#4361)

As in #4360, updates the uv project CLI to respect `.python-version`
files as default Python version requests. Additionally, updates project
interpreter discovery to fetch managed toolchains as in `uv venv
--preview`.

crates/uv-client/src/base_client.rs

@@ -32,7 +32,7 @@ pub struct BaseClientBuilder<'a> {
     keyring: KeyringProviderType,
     native_tls: bool,
     retries: u32,
-    connectivity: Connectivity,
+    pub connectivity: Connectivity,
     client: Option<Client>,
     markers: Option<&'a MarkerEnvironment>,
     platform: Option<&'a Platform>,

crates/uv-client/src/registry_client.rs

@@ -929,6 +929,16 @@ pub enum Connectivity {
     Offline,
 }
 
+impl Connectivity {
+    pub fn is_online(&self) -> bool {
+        matches!(self, Self::Online)
+    }
+
+    pub fn is_offline(&self) -> bool {
+        matches!(self, Self::Offline)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::str::FromStr;