[codex] migrate ExecutorFileSystem paths to PathUri (#27424)

## Why

We're moving exec-server to use PathUri for its internal path
representations.

## What

Move `ExecutorFileSystem` APIs to use `PathUri` instead of
`AbsolutePathBuf`. Future changes will convert higher-level parts of
exec-server.

codex-rs/exec-server/src/local_file_system_path_uri_tests.rs

@@ -0,0 +1,27 @@
+use codex_utils_path_uri::PathUri;
+use pretty_assertions::assert_eq;
+use tokio::io;
+
+use super::*;
+
+#[tokio::test]
+async fn direct_file_system_rejects_non_native_uri_as_invalid_input() {
+    let error = DirectFileSystem
+        .read_file(&non_native_uri(), /*sandbox*/ None)
+        .await
+        .expect_err("non-native URI should be rejected");
+
+    assert_eq!(error.kind(), io::ErrorKind::InvalidInput);
+}
+
+fn non_native_uri() -> PathUri {
+    #[cfg(unix)]
+    let uri = "file://server/share/file.txt";
+    #[cfg(windows)]
+    let uri = "file:///usr/local/file.txt";
+
+    match PathUri::parse(uri) {
+        Ok(uri) => uri,
+        Err(err) => panic!("valid non-native URI should parse: {err}"),
+    }
+}