PR 928

What's Changed

Enforces exactly 4 query expansions using structured output and aligned prompt instructions, improving consistency and recall behavior. Raises a clear error if the LLM returns the wrong count.

• Bug Fixes
  • Pydantic schema requires exactly 4 alternatives (min_items=max_items=4) via a shared constant.
  • Prompt updated to “exactly {number_of_expansions}” and wired to the same constant.
  • Post-validation length check with clearer error message to catch mismatches.