Preserve foreign paths in background terminal listings (#35850)

## Why

Background terminal working directories can use a path convention from a
different platform than the app-server host. Converting them to host-native
absolute paths can reject otherwise valid terminal entries and fail the list
request.

## What changed

- Represent `ThreadBackgroundTerminal.cwd` with `LegacyAppPathString` at the
  protocol boundary.
- Convert terminal `PathUri` values directly to their inferred native path
  strings instead of validating them against the host platform.

## Testing

Add protocol round-trip coverage for POSIX, Windows drive, and Windows UNC
working directories.

GitOrigin-RevId: d766d54bdc191e75955fb419bec6e60c65f52298

codex-rs/app-server-protocol/src/protocol/v2/thread.rs

@@ -1044,7 +1044,7 @@ pub struct ThreadBackgroundTerminal {
     pub item_id: String,
     pub process_id: String,
     pub command: String,
-    pub cwd: AbsolutePathBuf,
+    pub cwd: LegacyAppPathString,
     pub os_pid: Option<u32>,
     pub cpu_percent: Option<f64>,
     pub rss_kb: Option<u64>,