Deterministically order --find-links distributions (#16446)

Made to address this comment:
https://github.com/astral-sh/uv/pull/16103#discussion_r2437498249

This PR sorts the distributions collected by
`FlatIndexClient::read_from_directory` (used for `--find-links`) so
results are ordered deterministically by filename and index.

Cargo.lock

@@ -5668,6 +5668,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sys-info",
+ "tempfile",
  "thiserror 2.0.17",
  "tl",
  "tokio",

crates/uv-client/Cargo.toml

@@ -74,3 +74,4 @@ hyper-util = { workspace = true }
 insta = { workspace = true }
 tokio = { workspace = true }
 wiremock = { workspace = true }
+tempfile = { workspace = true }

crates/uv-client/src/flat_index.rs

@@ -321,6 +321,63 @@ impl<'a> FlatIndexClient<'a> {
                 index: flat_index.clone(),
             });
         }
+
+        dists.sort_by(|a, b| {
+            a.filename
+                .cmp(&b.filename)
+                .then_with(|| a.index.cmp(&b.index))
+        });
+
         Ok(FlatIndexEntries::from_entries(dists))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use fs_err::File;
+    use std::io::Write;
+    use tempfile::tempdir;
+
+    #[test]
+    fn read_from_directory_sorts_distributions() {
+        let dir = tempdir().unwrap();
+
+        let filenames = [
+            "beta-2.0.0-py3-none-any.whl",
+            "alpha-1.0.0.tar.gz",
+            "alpha-1.0.0-py3-none-any.whl",
+        ];
+
+        for name in &filenames {
+            let mut file = File::create(dir.path().join(name)).unwrap();
+            file.write_all(b"").unwrap();
+        }
+
+        let entries = FlatIndexClient::read_from_directory(
+            dir.path(),
+            &IndexUrl::parse(&dir.path().to_string_lossy(), None).unwrap(),
+        )
+        .unwrap();
+
+        let actual = entries
+            .entries
+            .iter()
+            .map(|entry| entry.filename.to_string())
+            .collect::<Vec<_>>();
+
+        let mut expected = filenames
+            .iter()
+            .map(|name| DistFilename::try_from_normalized_filename(name).unwrap())
+            .collect::<Vec<_>>();
+
+        expected.sort();
+
+        let expected = expected
+            .into_iter()
+            .map(|filename| filename.to_string())
+            .collect::<Vec<_>>();
+
+        assert_eq!(actual, expected);
+    }
+}