Preserve special filesystem subpaths as wire strings (#33107)

## Why

Special filesystem subpaths can contain relative or foreign-platform path
spellings. Treating this wire text as a host-native path can reinterpret it.

## What changed

- Store special filesystem subpaths as strings in the core permissions model.
- Represent app-server subpaths as `LegacyAppPathString` and convert them at
  the protocol boundary without changing their spelling.
- Add `LegacyAppPathString::from_string` for callers that already own legacy
  app-server path text.

GitOrigin-RevId: 2214f03aa997fd0e6eb2ab60d425304bb4bd6132

codex-rs/core/src/config/permissions.rs

@@ -274,7 +274,7 @@ fn insert_special_filesystem_permission_toml(
             insert_scoped_filesystem_permission_toml(
                 entries,
                 ":workspace_roots".to_string(),
-                subpath.unwrap_or_else(|| PathBuf::from(".")),
+                subpath.unwrap_or_else(|| ".".to_string()),
                 access,
             );
         }
@@ -303,7 +303,7 @@ fn insert_special_filesystem_permission_toml(
 fn insert_scoped_filesystem_permission_toml(
     entries: &mut BTreeMap<String, FilesystemPermissionToml>,
     path: String,
-    subpath: PathBuf,
+    subpath: String,
     access: FileSystemAccessMode,
 ) {
     let permission = entries
@@ -311,13 +311,10 @@ fn insert_scoped_filesystem_permission_toml(
         .or_insert_with(|| FilesystemPermissionToml::Scoped(BTreeMap::new()));
     match permission {
         FilesystemPermissionToml::Scoped(scoped_entries) => {
-            scoped_entries.insert(subpath.to_string_lossy().into_owned(), access);
+            scoped_entries.insert(subpath, access);
         }
         FilesystemPermissionToml::Access(_) => {
-            *permission = FilesystemPermissionToml::Scoped(BTreeMap::from([(
-                subpath.to_string_lossy().into_owned(),
-                access,
-            )]));
+            *permission = FilesystemPermissionToml::Scoped(BTreeMap::from([(subpath, access)]));
         }
     }
 }
@@ -608,7 +605,9 @@ fn compile_scoped_filesystem_path(
     }
 
     if let Some(special) = parse_special_path(path) {
-        let subpath = parse_relative_subpath(subpath)?;
+        let subpath = parse_relative_subpath(subpath)?
+            .to_string_lossy()
+            .into_owned();
         let special = match special {
             FileSystemSpecialPath::ProjectRoots { .. } => Ok(FileSystemPath::Special {
                 value: FileSystemSpecialPath::project_roots(Some(subpath)),
@@ -894,8 +893,7 @@ fn maybe_push_unknown_special_path_warning(
         startup_warnings,
         match subpath.as_deref() {
             Some(subpath) => format!(
-                "Configured filesystem path `{path}` with nested entry `{}` is not recognized by this version of Codex and will be ignored. Upgrade Codex if this path is required.",
-                subpath.display()
+                "Configured filesystem path `{path}` with nested entry `{subpath}` is not recognized by this version of Codex and will be ignored. Upgrade Codex if this path is required."
             ),
             None => format!(
                 "Configured filesystem path `{path}` is not recognized by this version of Codex and will be ignored. Upgrade Codex if this path is required."