tests: make test_walk_root deterministic

The mock server exposes the host's real filesystem, so assertions about
the contents of "/" depend on machine state: mock-ssh-server stats
every entry of a listing, so a single dangling symlink at the host root
fails the whole readdir, and fsspec's walk(on_error="omit") swallows
the error, silently turning the name/exists assertions into a no-op.

Assert only the yielded root, which distinguishes the fix (root "/")
from the regression (root "") on every machine. Also correct the
failure-mode note: most servers (e.g. OpenSSH) reject an empty path
with ENOENT outside REALPATH; servers implementing the SFTP draft's
empty-path rule resolve it to the default directory instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: shcheklein

tests/test_sshfs.py

@@ -221,13 +221,14 @@ def test_walk(fs, remote_dir):
 
 def test_walk_root(fs):
     # Regression: SSHFileSystem._strip_protocol("/") used to collapse the
-    # root to "", so walk("/") listed the home directory and yielded
-    # relative paths that were not considered to exist.
-    root, dirs, files = next(iter(fs.walk("/", maxdepth=1, detail=True)))
+    # root to "", so walk("/") resolved against the server's default
+    # directory (or failed outright on servers such as OpenSSH that
+    # reject empty paths outside REALPATH) and yielded root "" with
+    # names relative to it. Only the yielded root is asserted here: the
+    # mock server exposes the host's real filesystem, so the contents of
+    # "/" are not this test's to control.
+    root, _dirs, _files = next(fs.walk("/", maxdepth=1))
     assert root == "/"
-    for info in dirs.values():
-        assert info["name"].startswith("/")
-        assert fs.exists(info["name"])
 
 
 def test_strip_protocol():