fix: align indexing with other file backed ndarray implementations

[terraputix]

Fixes indexing to correctly handle clipped and empty slices across synchronous and asynchronous readers while preserving integer, slice, and ellipsis behavior. newaxis/None indexing was not correct and is removed entirely.

[Copilot]

Pull request overview

This PR fixes NumPy-style None/np.newaxis indexing for OmFileReader (and the async reader) by tracking NewAxis positions during index parsing and re-inserting size-1 dimensions after applying integer-index squeezing. It also adds a suite of Python tests for basic indexing semantics.

Changes:

• Extend ArrayIndex::get_ranges_and_squeeze_dims to return newaxis_dims in addition to read ranges and squeeze_dims.
• Update sync/async readers to apply newaxis_dims when reshaping the returned ndarray.
• Add Python indexing tests (integer/slice/ellipsis/negative slice/newaxis/mixed/errors) and additional Rust unit tests for NewAxis parsing.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
tests/test_read_write.py	Adds comprehensive indexing tests and NewAxis-specific assertions.
src/reader.rs	Plumbs newaxis_dims into array read/reshape path; updates an fsspec TypeError message.
src/reader_async.rs	Mirrors sync reader changes by passing newaxis_dims into async reshape logic.
src/array_index.rs	Enhances index parsing to track NewAxis positions and adjust squeeze/newaxis dims appropriately.

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