sandboxing: migrate cwd inputs to PathUri (#27816)

## Why

Sandbox cwd values can cross app-server and exec-server host boundaries.
They should retain URI semantics until the receiving host validates them
instead of being interpreted early as native paths.

## What

- Carry `PathUri` through filesystem sandbox contexts, sandbox commands,
and transform inputs.
- Convert command and policy cwd once in `SandboxManager::transform`,
then keep launch requests native.
- Preserve sandbox cwd over remote filesystem transport and reject
non-native URIs without fallback.
- Cache paired native/URI turn-environment cwd values during migration,
with immutable access to keep them synchronized.
- Extend existing protocol, forwarding, transform, and core runtime
tests.

codex-rs/core/src/thread_manager_tests.rs

@@ -648,12 +648,12 @@ async fn resume_and_fork_do_not_restore_thread_environments_from_rollout() {
         .expect("build resumed turn context");
     assert_eq!(resumed_turn.environments.turn_environments.len(), 1);
     assert_eq!(
-        resumed_turn.environments.turn_environments[0].cwd,
-        default_cwd
+        resumed_turn.environments.turn_environments[0].cwd(),
+        &default_cwd
     );
     assert_ne!(
-        resumed_turn.environments.turn_environments[0].cwd,
-        selected_cwd
+        resumed_turn.environments.turn_environments[0].cwd(),
+        &selected_cwd
     );
 
     let forked = manager
@@ -675,12 +675,12 @@ async fn resume_and_fork_do_not_restore_thread_environments_from_rollout() {
         .expect("build forked turn context");
     assert_eq!(forked_turn.environments.turn_environments.len(), 1);
     assert_eq!(
-        forked_turn.environments.turn_environments[0].cwd,
-        default_cwd
+        forked_turn.environments.turn_environments[0].cwd(),
+        &default_cwd
     );
     assert_ne!(
-        forked_turn.environments.turn_environments[0].cwd,
-        selected_cwd
+        forked_turn.environments.turn_environments[0].cwd(),
+        &selected_cwd
     );
 }