Reuse ValueIndexPair definition when declaring ValueStringHashes

[staging-devin-ai-integration]

This PR simplifies the type declaration by reusing the existing ValueIndexPair type alias.

Changes

• Replace std::vector<std::pair<std::size_t, std::size_t>> with std::vector<ValueIndexPair> in the ValueStringHashes type alias

Since ValueIndexPair is already defined as std::pair<std::size_t, std::size_t>, this makes the code more consistent and easier to maintain.

[staging-devin-ai-integration]

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