Show PyPy downloads during uv python list (#12915)

Fixes #12914.
When `PythonDownloadRequest` does not have the `implementation` set, do
not set it to CPython when calling `fill`, otherwise only CPython
interpreters are shown when listing interpreters available for download,
with `uv python list`.

crates/uv-python/src/downloads.rs

@@ -206,10 +206,7 @@ impl PythonDownloadRequest {
     /// Fill empty entries with default values.
     ///
     /// Platform information is pulled from the environment.
-    pub fn fill(mut self) -> Result<Self, Error> {
-        if self.implementation.is_none() {
-            self.implementation = Some(ImplementationName::CPython);
-        }
+    pub fn fill_platform(mut self) -> Result<Self, Error> {
         if self.arch.is_none() {
             self.arch = Some(Arch::from_env());
         }
@@ -222,6 +219,14 @@ impl PythonDownloadRequest {
         Ok(self)
     }
 
+    pub fn fill(mut self) -> Result<Self, Error> {
+        if self.implementation.is_none() {
+            self.implementation = Some(ImplementationName::CPython);
+        }
+        self = self.fill_platform()?;
+        Ok(self)
+    }
+
     /// Construct a new [`PythonDownloadRequest`] with platform information from the environment.
     pub fn from_env() -> Result<Self, Error> {
         Ok(Self::new(

crates/uv/src/commands/python/list.rs

@@ -79,16 +79,16 @@ pub(crate) async fn list(
             PythonListKinds::Downloads => Some(if all_platforms {
                 base_download_request
             } else {
-                base_download_request.fill()?
+                base_download_request.fill_platform()?
             }),
             PythonListKinds::Default => {
                 if python_downloads.is_automatic() {
                     Some(if all_platforms {
                         base_download_request
                     } else if all_arches {
-                        base_download_request.fill()?.with_any_arch()
+                        base_download_request.fill_platform()?.with_any_arch()
                     } else {
-                        base_download_request.fill()?
+                        base_download_request.fill_platform()?
                     })
                 } else {
                     // If fetching is not automatic, then don't show downloads as available by default