[codex] Add size to internal filesystem metadata (#27927)

## Why

`ExecutorFileSystem::get_metadata` reports file kind and timestamps but
not size. Internal callers that need to enforce a size limit therefore
have to read the complete file first, which is especially wasteful for
remote filesystems.

This adds the missing internal metadata so consumers can reject
oversized files before transferring or buffering them. The field is
named `size`, matching VS Code's `FileStat.size` filesystem convention.

## What changed

- add `size: u64` to internal `FileMetadata`
- populate it from the underlying filesystem metadata
- carry it through sandbox-helper and remote exec-server responses
- cover files, directories, symlink targets, and sandboxed reads across
local and remote filesystem implementations

The new field is intentionally not exposed through the app-server API.

## Testing

- `just test -p codex-exec-server get_metadata`
- `just test -p codex-exec-server
file_system_sandboxed_metadata_and_read_allow_readable_root`
- `just test -p codex-core-plugins`
- `just test -p codex-skills-extension`

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

@@ -103,6 +103,7 @@ impl FileSystemHandler {
             is_directory: metadata.is_directory,
             is_file: metadata.is_file,
             is_symlink: metadata.is_symlink,
+            size: metadata.size,
             created_at_ms: metadata.created_at_ms,
             modified_at_ms: metadata.modified_at_ms,
         })