OpenConceptLab/ocl_issues#2505 | added config for use_lexical_variants

[snyaggarwal]

Linked Issue

Refs OpenConceptLab/ocl_issues#2505

[paynejd]

This is the companion to oclapi2#868 — closes the loop on use_lexical_variants end to end (project persistence → checkbox → $match request). Reviewed together they now form a complete change.

The wiring is clean and minimal. Two small notes:

1. Cross-link missing in both PR bodies. Neither this PR's description nor oclapi2#868 mentions the sibling. Worth a one-line Companion: OpenConceptLab/oclapi2#868 in each so reviewers (and future archaeologists) see the pair.

2. "Use Lexical Variants" is opaque without context. Inline suggestion below — a short helper/description so even core users can tell what the checkbox does without digging through code.

Merge order doesn't matter: if this lands first, oclapi2 master ignores the unknown variants field; if oclapi2 lands first, oclmap master doesn't yet send it so default-disabled holds. No race.