fix: update cursor correctly in blob file API

[westonpace]

When read returns it should update the cursor to the end of the data just read. However, we were calculating this as end and not end - position.

The cursor is supposed to be relative to the start of the blob in the file (e.g. 0 == position).

[github-actions]

ACTION NEEDED
Lance follows the Conventional Commits specification for release automation.

The PR title and description are used as the merge commit message. Please update your PR title and description to match the specification.

For details on the error please inspect the "PR Title Check" action.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.25%. Comparing base (631e9bf) to head (8c78e8f).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/blob.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3018      +/-   ##
==========================================
+ Coverage   78.22%   78.25%   +0.02%     
==========================================
  Files         239      239              
  Lines       76576    76712     +136     
  Branches    76576    76712     +136     
==========================================
+ Hits        59902    60029     +127     
+ Misses      13626    13610      -16     
- Partials     3048     3073      +25     

Flag	Coverage Δ
unittests	78.25% <0.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.