Separate exec permission paths from core models (#33200)

## Why

Core filesystem permission models operate on native absolute paths, while exec
sandbox contexts must serialize paths as portable URIs.

## What changed

- Make the core filesystem permission and profile types concrete over
  `AbsolutePathBuf` instead of generic over their path representation.
- Add exec-specific permission types that convert native permission paths to
  `PathUri` values at the sandbox protocol boundary.
- Replace the legacy read/write-roots tuple with a named
  `LegacyReadWriteRoots` struct.

## Testing

- Add coverage that exec sandbox permission paths serialize as URIs and round
  trip through the filesystem protocol.

GitOrigin-RevId: a0ef5f8aba9bbfdf00a00fcc4f199314836bc85d

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

@@ -597,6 +597,11 @@ mod tests {
     use codex_file_system::FileSystemSandboxContext;
     use codex_network_proxy::ManagedNetworkSandboxContext;
     use codex_protocol::models::PermissionProfile;
+    use codex_protocol::permissions::FileSystemAccessMode;
+    use codex_protocol::permissions::FileSystemPath;
+    use codex_protocol::permissions::FileSystemSandboxEntry;
+    use codex_protocol::permissions::FileSystemSandboxPolicy;
+    use codex_protocol::permissions::NetworkSandboxPolicy;
     use codex_utils_path_uri::PathUri;
     use pretty_assertions::assert_eq;
     use std::collections::HashMap;
@@ -694,6 +699,38 @@ mod tests {
         .expect_err("native absolute sandbox cwd should not deserialize as a URI");
     }
 
+    #[test]
+    fn filesystem_protocol_round_trips_permission_paths_as_uris() {
+        let native_cwd = std::env::current_dir().expect("current directory");
+        let cwd = PathUri::from_host_native_path(&native_cwd).expect("cwd URI");
+        let mut file_system_policy =
+            FileSystemSandboxPolicy::restricted(vec![FileSystemSandboxEntry {
+                path: FileSystemPath::Path {
+                    path: native_cwd.try_into().expect("absolute cwd"),
+                },
+                access: FileSystemAccessMode::Read,
+            }]);
+        file_system_policy.glob_scan_max_depth = Some(2);
+        let permissions = PermissionProfile::from_runtime_permissions(
+            &file_system_policy,
+            NetworkSandboxPolicy::Restricted,
+        );
+        let sandbox =
+            FileSystemSandboxContext::from_permission_profile_with_cwd(permissions, cwd.clone());
+
+        let serialized = serde_json::to_value(&sandbox).expect("serialize sandbox");
+
+        assert_eq!(
+            serialized["permissions"]["file_system"]["entries"][0]["path"]["path"],
+            serde_json::json!(cwd.to_string())
+        );
+        assert_eq!(
+            serde_json::from_value::<FileSystemSandboxContext>(serialized)
+                .expect("deserialize sandbox"),
+            sandbox
+        );
+    }
+
     #[test]
     fn http_request_timeout_treats_omitted_and_null_as_no_timeout() {
         let omitted: HttpRequestParams = serde_json::from_value(serde_json::json!({