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/exec-server/src/protocol.rs

@@ -450,6 +450,8 @@ mod base64_bytes {
 mod tests {
     use super::FsReadFileParams;
     use super::HttpRequestParams;
+    use crate::FileSystemSandboxContext;
+    use codex_protocol::models::PermissionProfile;
     use codex_utils_path_uri::PathUri;
     use pretty_assertions::assert_eq;
 
@@ -458,14 +460,22 @@ mod tests {
         let legacy_path = std::env::current_dir()
             .expect("current directory")
             .join("legacy-file.txt");
+        let legacy_cwd = std::env::current_dir().expect("current directory");
+        let expected_sandbox = FileSystemSandboxContext::from_permission_profile_with_cwd(
+            PermissionProfile::default(),
+            PathUri::from_path(&legacy_cwd).expect("cwd URI"),
+        );
+        let mut legacy_sandbox =
+            serde_json::to_value(&expected_sandbox).expect("sandbox should serialize");
+        legacy_sandbox["cwd"] = serde_json::json!(legacy_cwd.to_string_lossy());
         let params: FsReadFileParams = serde_json::from_value(serde_json::json!({
             "path": legacy_path.to_string_lossy(),
-            "sandbox": null,
+            "sandbox": legacy_sandbox,
         }))
         .expect("legacy absolute path should deserialize");
         let expected = FsReadFileParams {
             path: PathUri::from_path(legacy_path).expect("path URI"),
-            sandbox: None,
+            sandbox: Some(expected_sandbox.clone()),
         };
 
         assert_eq!(params, expected);
@@ -473,7 +483,8 @@ mod tests {
             serde_json::to_value(params).expect("params should serialize"),
             serde_json::json!({
                 "path": expected.path.to_string(),
-                "sandbox": null,
+                "sandbox": serde_json::to_value(expected_sandbox)
+                    .expect("sandbox should serialize"),
             })
         );
     }