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/core/src/tools/handlers/apply_patch_tests.rs

@@ -290,7 +290,7 @@ fn write_permissions_for_paths_keep_dirs_outside_workspace_root() {
         permissions
             .and_then(|profile| profile.file_system)
             .and_then(|fs| fs.legacy_read_write_roots())
-            .and_then(|(_read, write)| write),
+            .and_then(|roots| roots.write),
         Some(vec![expected_outside])
     );
 }