fix(query-engine): reconcile CountMinSketchWithHeap merge semantics

[milindsrivastava1997]

Parent

#678

What to build

Define and enforce one consistent contract for CountMinSketchWithHeap merging across planning and execution. The planner currently treats heap top-k lists as non-mergeable, while the query engine merges heap accumulators across windows.

Choose one supported behavior: prohibit these merges, support them with documented approximation guarantees, or change planning capabilities to reflect the runtime implementation.

Acceptance criteria

• The supported merge behavior is explicitly documented.
• Planner mergeability flags and query-engine behavior agree.
• Range/window execution either follows the contract or fails clearly.
• Tests cover multiple-window heap top-k behavior and the chosen accuracy/limitation contract.

Blocked by

None - can start immediately.

Concrete example

The planner currently declares:

AggregationType::CountMinSketchWithHeap => p(false, false, true)

But the query engine can execute:

let merged = left.merge_with(&right)?;

A range query spanning two precompute windows therefore has to choose between:

• rejecting the merge because the planner says heap top-k is non-mergeable; or
• merging the heaps and documenting that global top-k accuracy is approximate.

The issue should establish which behavior is authoritative and add a two-window example that verifies it.