feat: move file name derivation into codex-file-search (#8334)

## Summary

  - centralize file name derivation in codex-file-search
  - reuse the helper in app-server fuzzy search to avoid duplicate logic
  - add unit tests for file_name_from_path

  ## Testing

  - cargo test -p codex-file-search
  - cargo test -p codex-app-server

codex-rs/Cargo.lock

@@ -1454,6 +1454,7 @@ dependencies = [
  "clap",
  "ignore",
  "nucleo-matcher",
+ "pretty_assertions",
  "serde",
  "serde_json",
  "tokio",

codex-rs/app-server/src/fuzzy_file_search.rs

@@ -1,6 +1,5 @@
 use std::num::NonZero;
 use std::num::NonZeroUsize;
-use std::path::Path;
 use std::path::PathBuf;
 use std::sync::Arc;
 use std::sync::atomic::AtomicBool;
@@ -63,11 +62,7 @@ pub(crate) async fn run_fuzzy_file_search(
             Ok(Ok((root, res))) => {
                 for m in res.matches {
                     let path = m.path;
-                    //TODO(shijie): Move file name generation to file_search lib.
-                    let file_name = Path::new(&path)
-                        .file_name()
-                        .map(|name| name.to_string_lossy().into_owned())
-                        .unwrap_or_else(|| path.clone());
+                    let file_name = file_search::file_name_from_path(&path);
                     let result = FuzzyFileSearchResult {
                         root: root.clone(),
                         path,

codex-rs/file-search/Cargo.toml

@@ -20,3 +20,6 @@ nucleo-matcher = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 tokio = { workspace = true, features = ["full"] }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }

codex-rs/file-search/src/lib.rs

@@ -40,6 +40,14 @@ pub struct FileMatch {
     pub indices: Option<Vec<u32>>, // Sorted & deduplicated when present
 }
 
+/// Returns the final path component for a matched path, falling back to the full path.
+pub fn file_name_from_path(path: &str) -> String {
+    Path::new(path)
+        .file_name()
+        .map(|name| name.to_string_lossy().into_owned())
+        .unwrap_or_else(|| path.to_string())
+}
+
 #[derive(Debug)]
 pub struct FileSearchResults {
     pub matches: Vec<FileMatch>,
@@ -403,6 +411,7 @@ fn create_pattern(pattern: &str) -> Pattern {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use pretty_assertions::assert_eq;
 
     #[test]
     fn verify_score_is_none_for_non_match() {
@@ -434,4 +443,14 @@ mod tests {
 
         assert_eq!(matches, expected);
     }
+
+    #[test]
+    fn file_name_from_path_uses_basename() {
+        assert_eq!(file_name_from_path("foo/bar.txt"), "bar.txt");
+    }
+
+    #[test]
+    fn file_name_from_path_falls_back_to_full_path() {
+        assert_eq!(file_name_from_path(""), "");
+    }
 }