fix: avoid empty range reads for zero-length blobs

[Xuanwo]

Blob reads should return empty bytes when the logical blob is empty or the cursor is already at EOF. Today BlobFile::read / read_up_to can still issue a get_range(start..end) request with start == end, which is tolerated by local readers but rejected by cloud object stores.

This showed up while investigating random_blob failures on the original-scale laion10m-full dataset, where legacy blob reads on S3 failed with errors like Range started at 1 and ended at 1. The fix short-circuits empty reads and restores the cursor to blob-relative semantics after read(), and adds regression coverage for both the empty-range case and packed-blob cursor behavior.

[github-actions]

PR Review

Clean, well-motivated bugfix. The core changes are correct:

1. read() cursor fix — The old code set the cursor to position + size (an absolute file offset), but do_with_reader stores the value as a blob-relative cursor. This meant a second read() on a packed blob would compute start = position + (position + size), reading from the wrong offset. Returning size is correct.

2. Empty-range short-circuits — Correctly avoids issuing get_range(n..n) which cloud stores reject.

One minor note: In read_up_to, the second guard if read_size == 0 (after computing len.min((size - cursor) as usize)) is unreachable — the first guard already ensures cursor < size (so size - cursor >= 1) and len > 0. Not a bug, just dead code. Up to you whether to remove it.

Tests look good — the RejectEmptyRangeObjectStore mock directly reproduces the S3 failure mode, and test_blob_file_read_tracks_relative_cursor validates the cursor semantics on a real file.

LGTM

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/blob.rs	50.00%	29 Missing and 2 partials ⚠️

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