planner/optimizer: dedupe extract_requirements vs build_query_requirements_promql

[milindsrivastava1997]

planner/optimizer — `aqe_extractor.rs::extract_requirements` (around line 298) duplicates `SimplePromQLEngine::build_query_requirements_promql` in the query-engine crate (`asap-query-engine/src/engines/simple_engine/promql.rs:614`). The author's own TODO comment already flags this.

`build_query_requirements_promql` is currently a private `&self` method tied to `SimplePromQLEngine`. The shared logic should be extracted into a free function (likely in `asap_types::query_requirements`) and called from both sites.

Found during code review of PR #407.