`os.chmod` is missing validation for `dir_fd` and `follow_symlinks` when `path` is a file descriptor

Bug report

Bug description:

The documentation for os.chmod says it is not correct to use dir_fd or follow_symlinks when path is an open file descriptor...

https://github.com/python/cpython/blob/f74cdf80a120649e4c353430da8cbd1305c00993/Modules/posixmodule.c#L4092-L4095

... but os_chmod_impl never checks it.

When path->is_fd is true it takes the fchmod path and returns success, so both keywords are ignored altogether:

https://github.com/python/cpython/blob/f74cdf80a120649e4c353430da8cbd1305c00993/Modules/posixmodule.c#L4146-L4149

Other os utils (I checked os.stat, os.chown, and os.utime) reject this combination with dir_fd_and_fd_invalid / fd_and_follow_symlinks_invalid before the syscall. os.chown for example:

https://github.com/python/cpython/blob/f74cdf80a120649e4c353430da8cbd1305c00993/Modules/posixmodule.c#L4548-L4550

(The Windows path also has the same problem with: path->is_fd going directly to win32_fchmod.)

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-156265

从 Modules/posixmodule.c 中的 os_chmod_impl 开始，检查 POSIX 和 Windows 的 path->is_fd 分支。将它们的处理方式与 os.chown、os.stat 和 os.utime 使用的验证进行比较。当 path 是文件描述符时，拒绝无效的 dir_fd 和 follow_symlinks 组合，并且相关 os 测试覆盖了这一行为，即表示完成。