fix(pgvector): order retrieval by distance operator

[2830500285]

Pgvector currently computes Haystack-facing scores correctly, but then orders embedding retrieval by the score alias. For cosine similarity that turns the pgvector distance operator into an expression and orders it descending, so HNSW/IVFFlat indexes cannot satisfy the ORDER BY shape pgvector expects.

This keeps the selected score column unchanged, while sorting by the raw pgvector distance operator in ascending order. The result ordering stays the same for cosine similarity and inner product, l2_distance remains distance-ascending, and Postgres can use the vector index for the retrieval sort.

SELECT *, 1 - (embedding <=> '[...]') AS score
FROM schema.table
ORDER BY embedding <=> '[...]' ASC
LIMIT 5

Fixes #2199

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Coverage report (pgvector)

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/pgvector/src/haystack_integrations/document_stores/pgvector
document_store.py					1371
Project Total

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[julian-risch]

Looks good to me and I ran integration tests locally. Thank you for this contribution @2830500285 !