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",