fix: honor index_path when input_dir is a local directory

[javierdejesusda]

Before submitting

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

What does this PR do?

Fixes #800.

When input_dir points to a local directory, a user-provided index_path was ignored and loading failed. In dataset_utilities.py the branch that copies index.json from index_path into the cache directory was nested inside isinstance(input_dir.url, str), so it only ran for remote inputs. For a local input_dir the url is None, the branch was skipped, the index was never materialized into the cache path, and the dataset could not resolve.

The fix checks index_path first, independent of whether the input is local or remote: if index.json is not already cached and an explicit index_path was provided, copy it into the cache path. The existing remote-download branch is preserved for the case where no index_path is given.

Added test_subsample_streaming_dataset_with_local_index_path, which uses a local input_dir with no index.json plus an index_path in a separate directory, and asserts that subsampling resolves correctly and the index is materialized into the local dataset directory. It fails on main and passes with this change.

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Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81%. Comparing base (5201360) to head (ed371c0).
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Additional details and impacted files

@@         Coverage Diff         @@
##           main   #839   +/-   ##
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  Coverage    81%    81%           
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  Files        54     54           
  Lines      7671   7671           
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+ Hits       6186   6191    +5     
+ Misses     1485   1480    -5     

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