Function parse_per_table_semantic_index 

pub fn parse_per_table_semantic_index(
    ctx: &QueryContextRef,
) -> Option<PerTableSemanticIndex>

Parses the per-table semantic index off the context extension. Call once per create-planning round: a first write creating N tables would otherwise re-parse the whole index N times. None when the request carries no index (logs, traces, Prom RW v1) or it fails to parse.