fix: strip trailing separator in FileSystem.is_writeable - #4300

Summary

FileSystem.is_writeable() calls path.rstrip("/\\") but discards the result, so the trailing separator is never actually stripped:

touch_filename = _WRITE_TEST_FILENAME if is_azure_path(path) else uuid()
path.rstrip("/\\")                                  # no-op: result discarded
touch_file = f"{path}{self.fs.sep}{touch_filename}"

FileRecorder.is_writeable() passes the user's log_dir straight through, and log dirs are commonly given with a trailing slash (--log-dir ./logs/, s3://bucket/logs/). With the trailing slash left in place, the write-test path becomes s3://bucket/logs//.inspect_write_test — a double separator, which is a malformed key on S3/Azure.

Fix

Assign the stripped value back:

path = path.rstrip("/\\")

(The module already has a strip_trailing_sep() helper; this just makes the existing inline strip actually take effect.)

Test

Added test_is_writeable_strips_trailing_sep in tests/util/test_file.py: it calls is_writeable("mydir/") on an in-memory filesystem (with touch/rm patched) and asserts the write-test path contains no double separator. It fails on main (assert '//' not in 'mydir//...') and passes with this change.

Verified locally with pytest tests/util/test_file.py::test_is_writeable_strips_trailing_sep and ruff check / ruff format --check (clean). A few unrelated path/URI tests in that file fail only on a Windows checkout due to POSIX-path assumptions; they're untouched by this change and pass on CI.