[diskann/disk] Remove flat_search from DiskANNIndex

[hildebrandmw]

Remove flat_search from DiskANNIndex and the IdIterator trait from diskann. Since the only caller was from diskann-disk, add a new flat_search inherent method to DiskIndexSearcher.

The flat search method is not compatible with the experimental direction in #1067 and with #983 on the horizon, this is safe to move for now.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.50%. Comparing base (f4757a3) to head (0cec906).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1076      +/-   ##
==========================================
- Coverage   89.51%   89.50%   -0.01%     
==========================================
  Files         459      459              
  Lines       85646    85565      -81     
==========================================
- Hits        76663    76583      -80     
+ Misses       8983     8982       -1     

Flag	Coverage Δ
miri	89.50% <100.00%> (-0.01%)	⬇️
unittests	89.12% <100.00%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
diskann-disk/src/search/provider/disk_provider.rs	91.19% <100.00%> (+0.31%)	⬆️
diskann/src/graph/glue.rs	96.31% <ø> (ø)
diskann/src/graph/index.rs	95.31% <ø> (-0.14%)	⬇️
diskann/src/graph/test/cases/index.rs	98.01% <ø> (-0.02%)	⬇️
diskann/src/graph/test/provider.rs	95.06% <ø> (-0.02%)	⬇️

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