Update FlashRank to use only unique documents

[svilupp]

• Changed behavior of RAGTools.rerank(::FlashRanker,...) to always dedupe input chunks (to reduce compute requirements).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 14.28571% with 12 lines in your changes missing coverage. Please review.

Project coverage is 92.37%. Comparing base (4c6a4d0) to head (5d9805a).

Files	Patch %	Lines
ext/FlashRankPromptingToolsExt.jl	0.00%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #166 +/- ## ========================================== - Coverage 92.52% 92.37% -0.15% ========================================== Files 44 44 Lines 3614 3622 +8 ========================================== + Hits 3344 3346 +2 - Misses 270 276 +6 ```

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