fix: panic if only one partition and split is triggered

[BubbleCal]

• handle the case of single partition is split
• this also fixes that single partition won't count the new data while checking split threshold
• rewrite the tests to cover more cases

[BubbleCal]

We can't know how many rows in the reader, but SPFresh needs num_rows to determine whether to trigger a split job, so remove this

[BubbleCal]

This would slow down indexing if num_partitions=1, but that means the dataset is small (otherwise users should indexing with more partitions), so it's fine

[BubbleCal]

here min_dist is not f32::INFINITY so min_dist_idx must be Some(...)

[BubbleCal]

This is a join job, which means there are at least 2 partitions, so min_dist_idx won't be None

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.69725% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.24%. Comparing base (8c6732e) to head (111b367).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance/src/index/vector/ivf/v2.rs	91.09%	17 Missing ⚠️
rust/lance/src/index/vector/ivf.rs	0.00%	3 Missing ⚠️
rust/lance-index/src/vector/hnsw/index.rs	0.00%	2 Missing ⚠️
rust/lance/src/index/vector/fixture_test.rs	0.00%	2 Missing ⚠️
rust/lance/src/index/vector/pq.rs	0.00%	2 Missing ⚠️
rust/lance/src/session/index_extension.rs	0.00%	2 Missing ⚠️
rust/lance/src/index/vector/builder.rs	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5241      +/-   ##
==========================================
+ Coverage   82.23%   82.24%   +0.01%     
==========================================
  Files         344      344              
  Lines      144755   144774      +19     
  Branches   144755   144774      +19     
==========================================
+ Hits       119033   119067      +34     
+ Misses      21800    21788      -12     
+ Partials     3922     3919       -3     

Flag	Coverage Δ
unittests	82.24% <86.69%> (+0.01%)	⬆️

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