Fail renaming a file to or from a path with a trailing separator

[kratos0718]

Describe the changes

Under Posix, rename() dropped the trailing separator in absnormpath before checking anything, so for a regular file foo:

• os.rename("foo", "bar/") renamed the file to bar (the real call fails and leaves foo in place)
• os.rename("foo", "foo/") succeeded as a no-op
• os.rename("foo/", "bar") renamed the file

A trailing separator requires a directory. When the source is not a directory and the parent of the target exists, Linux raises ENOTDIR, while macOS raises ENOENT for a missing target, EISDIR for an existing directory and ENOTDIR otherwise. A trailing separator on a regular source file raises ENOTDIR on both. Windows is unchanged.

I ran the new tests with TEST_REAL_FS=1 on macOS and as a non-root user on Linux (Debian, python 3.13), and they fail without the change. The existing trailing-separator tests for symlinks and directories still pass.

Tasks

• Unit tests added that reproduce the issue or prove feature is working
• Fix or feature added
• Entry to release notes added
• Pre-commit CI shows no errors
• Unit tests passing
• For documentation changes: The Read the Docs preview builds and looks as expected

[mrbean-bremen]

Thanks! I'm traveling right now, will review when I get back (in a bit over a week).

[davidlbaird]

fake_filesystem_vs_real_test.py should also be updated to validate the fake_filesystem behavior against os on the host system.

[davidlbaird]

Please also create an issue, that will be referenced in CHANGES.md for the next release.