[release-branch.go1.25] os: avoid escape from Root via ReadDir or Readdir

[release-branch.go1.25] os: avoid escape from Root via ReadDir or Readdir

When reading the contents of a directory using
File.ReadDir or File.Readdir, the os.FileInfo was
populated on Unix platforms using lstat.
This lstat call is vulnerable to a TOCTOU race
and could escape the root.

For example:
  - Open the directory "dir" within a Root.
    This directory contains a file named "file".
  - Use File.ReadDir to list the contents of "dir",
    receiving a os.DirEntry for "dir/file".
  - Replace "dir" with a symlink to "/etc".
  - Use DirEntry.Info to retrieve the FileInfo for "dir/file".
    This FileInfo contains information on "/etc/file" instead.

This escape permits identifying the presence or absence of
files outside a Root, as well as retreiving stat metadata
(size, mode, modification time, etc.) for files outside a Root.

This escape does not permit reading or writing to files
outside a Root.

For #77827
Fixes #77833
Fixes CVE-2026-27139

Change-Id: I40004f830c588e516aff8ee593d630d36a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/749480
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>
(cherry picked from commit 657ed934e85dc575aad51356c4b437961e7c1313)
Reviewed-on: https://go-review.googlesource.com/c/go/+/749920