test: add additional test cases for FindIndex method in FastSearcher

[siriak]

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[siriak]

Fixes #572

[Copilot]

Pull Request Overview

This PR adds comprehensive test coverage for the FindIndex method in FastSearcher by introducing a new test method with multiple test cases that cover various scenarios of the hybrid binary-interpolation search algorithm.

Key changes:

• Added 10 new deterministic test cases covering edge cases and different distributions
• Tests specifically target the algorithm's behavior with left/right segments, boundary conditions, and various data distributions

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.96%. Comparing base (5aee965) to head (7f9aab2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #578   +/-   ##
=======================================
  Coverage   96.96%   96.96%           
=======================================
  Files         300      300           
  Lines       12598    12598           
  Branches     1842     1842           
=======================================
  Hits        12216    12216           
  Misses        242      242           
  Partials      140      140           

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