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/agents_md.rs

@@ -57,7 +57,7 @@ pub(crate) async fn load_project_instructions(
             config,
             filesystem.as_ref(),
             &turn_environment.environment_id,
-            &turn_environment.cwd,
+            turn_environment.cwd(),
         )
         .await
         {