fix: avoid trailing filesystem directory probes

[fengjikui]

Description

Fixes #3866.

FilesystemClient.dataset_path and FilesystemClient.get_table_dir() are passed directly to fsspec directory probes such as isdir() and exists(). They currently force a trailing separator, which is benign on some backends but can fail on stricter implementations such as OneLake/Fabric.

This keeps the existing table-prefix behavior for layouts that need folder-style prefixes, but returns dataset/table directory paths without a trailing separator before those paths are probed or converted to remote URLs.

Validation

env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy uv run --with pyparsing pytest \
  'tests/load/filesystem/test_filesystem_client.py::test_filesystem_dirs_do_not_have_trailing_separators[data-{table_name}/{load_id}.{file_id}.{ext}]' \
  'tests/load/filesystem/test_filesystem_client.py::test_filesystem_dirs_do_not_have_trailing_separators[data-{table_name}.{load_id}.{file_id}.{ext}]' -q
# 2 passed

env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy uv run --with pyparsing pytest \
  tests/load/filesystem/test_filesystem_client.py::test_list_dlt_table_files_with_separator_in_pipeline_name -q
# 5 passed

env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy uv run --with pyparsing pytest \
  'tests/load/filesystem/test_filesystem_client.py::test_successful_load[data-{table_name}/{load_id}.{file_id}.{ext}-replace]' \
  'tests/load/filesystem/test_filesystem_client.py::test_successful_load[memory:///m-{table_name}/{load_id}.{file_id}.{ext}-replace]' -q
# 2 passed

env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy uv run ruff check \
  dlt/destinations/impl/filesystem/filesystem.py \
  tests/load/filesystem/test_filesystem_client.py
# All checks passed!

env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy uv run python -m py_compile \
  dlt/destinations/impl/filesystem/filesystem.py \
  tests/load/filesystem/test_filesystem_client.py
# passed

git diff --check
# passed

I also tried the full parametrized test_filesystem_dirs_do_not_have_trailing_separators locally. The local data cases passed; remote bucket cases require credentials or optional filesystem extras that are not present in my local environment.