Elastic Avg: engine queries Rate but planner precomputes Sum+Count

[milindsrivastava1997]

Bug: Mismatch between planner and engine for Elastic Avg aggregation.

Planner (planner/elastic_dsl.rs:157-162) precomputes [Sum, Count] sketches.
Engine (simple_engine/elastic.rs:122) queries Statistic::Rate at runtime.

Engine will miss precomputed data or hit the wrong accumulator for any Elastic Avg query.

Fix: Engine should query Sum and Count and divide, matching the planner.

[milindsrivastava1997]

Fixed in this branch by returning None from build_query_metadata_elastic for Avg queries (elastic.rs), which causes graceful fallback to Prometheus — the same behavior SQL and PromQL already have for Avg (SQL's parse_single_statistic returns None when the statistic maps to multiple values; PromQL's context builder returns None when statistics_to_compute.len() != 1).

The previous code used Statistic::Rate for Avg, which was completely wrong — the planner precomputes [Sum, Count] (two separate MultipleSum accumulators), not a rate accumulator. This would have returned incorrect results rather than falling back.

Properly serving Avg requires querying both the Sum and Count accumulators then dividing — that's a deeper engine change tracked in a follow-up issue.