refactor(query-engine): dedup instant/range PromQL arm-building and parsing in promql.rs

[milindsrivastava1997]

High-level overview of the refactor (asap-query-engine/src/engines/simple_engine/promql.rs):

1. detect_scalar_arm — new free function extracting the scalar-vs-vector detection logic that was byte-for-byte duplicated in handle_binary_expr_promql and handle_binary_expr_range_promql.
2. resolve_arm_leaf_context — new shared method extracting the Paren-unwrap recursion + structural config lookup + base QueryExecutionContext build that was duplicated between build_arm_logical_plan (instant) and build_arm_range_context (range). Each builder keeps its
   own arm-combination logic on top (DataFusion plan join vs. sample-level timestamp join) since those genuinely differ.
3. Parse-once wiring — added private build_query_execution_context_from_parsed and build_range_query_execution_context_from_parsed, taking an already-parsed AST. The public String-taking functions became thin wrappers (unchanged signatures, so none of the ~15 test
   call sites needed touching). handle_query_promql/handle_range_query_promql now parse the query exactly once and pass that AST straight through, instead of parsing again inside the context builders they call.
4. New tests (range_query_arithmetic_tests.rs) — added vector-vector and scalar range-query arithmetic coverage, since nothing previously exercised handle_range_query_promql's binary path at all.