Function pickEmbedTextForHyde

v3.1.0 — pick the text that should be embedded for an embeddings-search call. HyDE-augmented retrieval prefers the agent-supplied hypothetical_answer (Gao et al 2023); falls back to the raw query when that's absent / empty / whitespace-only.

Pure helper so we can unit-test the decision in isolation (the real embeddingsSearch function loads the @huggingface/transformers embedder, which is out of scope for unit tests).