fix: full text search index broken after optimize_indices()

[BubbleCal]

this can be reproduced by optimizing FTS without any new data.
the existing tokens could be reordered then the offsets broken, leads to the index would return random results.
existing tests only verify whether the new data can be queried, so we didn't find this bug.
added a test to query existing data

[github-actions]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 77.98%. Comparing base (71f323a) to head (7140241).

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/inverted/index.rs	77.77%	0 Missing and 2 partials ⚠️
rust/lance-index/src/scalar/inverted/builder.rs	87.50%	0 Missing and 1 partial ⚠️
rust/lance/src/index.rs	98.41%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3145      +/-   ##
==========================================
+ Coverage   77.93%   77.98%   +0.05%     
==========================================
  Files         242      242              
  Lines       81736    81798      +62     
  Branches    81736    81798      +62     
==========================================
+ Hits        63698    63794      +96     
+ Misses      14849    14812      -37     
- Partials     3189     3192       +3     

Flag	Coverage Δ
unittests	77.98% <95.00%> (+0.05%)	⬆️

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[wjones127]

If there's no new data, does optimize indices do anything? Will it in the future? Should you append some new data before running this?

[BubbleCal]

it's possible, like merging existing indices. but i think it should do nothing for scalar index & FTS, because they always have only 1 delta index. added adding new data here and make the tests cover more cases