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])
     );
 }

codex-rs/core/tests/suite/request_permissions.rs

@@ -1649,7 +1649,10 @@ async fn partial_request_permissions_grants_do_not_preapprove_new_permissions()
     let approval_file_system = approval_permissions
         .file_system
         .expect("expected filesystem permissions");
-    let (approval_reads, approval_writes) = approval_file_system
+    let codex_protocol::models::LegacyReadWriteRoots {
+        read: approval_reads,
+        write: approval_writes,
+    } = approval_file_system
         .legacy_read_write_roots()
         .expect("expected legacy-compatible permissions");
     assert!(approval_reads.as_ref().is_none_or(Vec::is_empty));
@@ -1657,7 +1660,10 @@ async fn partial_request_permissions_grants_do_not_preapprove_new_permissions()
     let mut approval_writes = approval_writes.unwrap_or_default();
     approval_writes.sort_by_key(|path| path.display().to_string());
 
-    let (_, expected_writes) = merged_permissions
+    let codex_protocol::models::LegacyReadWriteRoots {
+        write: expected_writes,
+        ..
+    } = merged_permissions
         .file_system
         .expect("expected merged filesystem permissions")
         .legacy_read_write_roots()