fix: forward storage_options as kwargs in CloudParquetDir

[SAY-5]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
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• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes #835.

CloudParquetDir unpacked storage_options positionally with fsspec.filesystem(provider, *self.storage_options), which passes the dict keys as positional arguments and raises TypeError: filesystem() takes 1 positional argument but ... were given whenever a non-empty storage_options is supplied (e.g. S3 credentials). Switching to **self.storage_options forwards them as keyword arguments, matching how litdata/raw/indexer.py already calls fsspec.filesystem.

Added a regression test asserting the storage options reach fsspec.filesystem as keyword arguments.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81%. Comparing base (82a9c3c) to head (3b6b036).
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Additional details and impacted files

@@         Coverage Diff         @@
##           main   #836   +/-   ##
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  Coverage    81%    81%           
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  Files        54     54           
  Lines      7671   7671           
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  Hits       6186   6186           
  Misses     1485   1485           

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