fix: postfilter should be applied in full if knn/fts even if scalar index applies

[westonpace]

If we are in a post-filtering situation (ann/knn/fts) and there is a scalar index that can be applied to the filter then the scalar index part was being skipped.

This also fixes a bug where we would consider btree/bitmap indices when trying to auto-pick the FTS column.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes missing coverage. Please review.

Project coverage is 77.79%. Comparing base (47eb00e) to head (8cb11e0).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/scanner.rs	38.46%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2931      +/-   ##
==========================================
+ Coverage   77.75%   77.79%   +0.03%     
==========================================
  Files         231      231              
  Lines       70485    70593     +108     
  Branches    70485    70593     +108     
==========================================
+ Hits        54805    54915     +110     
- Misses      12769    12775       +6     
+ Partials     2911     2903       -8     

Flag	Coverage Δ
unittests	77.79% <52.94%> (+0.03%)	⬆️

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