fix(multiquery): forward retrieve options to the underlying retriever

[serhiizghama]

The multi-query retriever drops any options passed to Retrieve. In multiQueryRetriever.Retrieve each RetrieveTask is built without RetrieveOptions, so WithTopK, WithScoreThreshold, WithEmbedding, WithDSLInfo etc. never reach the underlying retriever — the per-call knobs silently do nothing and you're stuck with whatever defaults the wrapped retriever was constructed with.

The router retriever handles the same fan-out correctly (flow/retriever/router/router.go sets RetrieveOptions: opts on its tasks), so this is just a missed forward on the multiquery path. Fixed by passing opts through, matching the router.

Added a test that calls Retrieve with WithTopK(7) and asserts the underlying retriever actually receives it. go test ./flow/retriever/multiquery/ -race passes; without the one-line change the new test fails.

[CLAassistant]

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