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/context/environment_context.rs

@@ -46,7 +46,7 @@ impl EnvironmentContextEnvironment {
             .iter()
             .map(|environment| Self {
                 id: environment.environment_id.clone(),
-                cwd: environment.cwd.clone(),
+                cwd: environment.cwd().clone(),
                 shell: environment
                     .shell
                     .as_ref()