Account for retained sort-key capacity

[kosiew]

Parent: #23393

Context: PR comment

Goal

Make partial group-ordering memory accounting include the retained allocation
for its Vec<ScalarValue> sort key.

Invariant / contract

GroupOrderingPartial::heap_size() reports the retained heap allocations owned
by its state. For an in-progress state, this includes the sort-key vector's
allocated capacity and heap allocations owned by its initialized scalar
elements, each exactly once. It does not add an inline descriptor already
charged by GroupOrdering::size().

Tasks

• Replace the live-element-only sort-key accounting in
  State::heap_size() with capacity-aware DFHeapSize accounting, following
  the grouped COUNT accumulator's established approach.
• Add a deterministic test with spare Vec<ScalarValue> capacity and a
  heap-owning scalar, proving spare slots and initialized scalar payloads are
  accounted for without relying on allocator measurements.
• Preserve the existing ownership boundary: do not charge inline
  descriptors or allocations owned by other ordering state again.

Acceptance criteria

• An in-progress sort key with spare vector capacity reports that capacity
  even when the vector's live length is smaller.
• Heap allocations owned by initialized ScalarValues remain included.
• Existing start/complete and ordering-state accounting behavior remains
  unchanged.

Tests / verification

• Add or update the focused GroupOrderingPartial tests in
  datafusion/physical-plan/src/aggregates/order/partial.rs.
• Run cargo test -p datafusion-physical-plan.
• Run ./ci/scripts/doc_prettier_check.sh --write --allow-dirty for this
  Markdown change.

Dependencies / blockers

Keep the descriptor accounting established by the GroupOrdering owner
separate from this retained-capacity fix.

[mohitgurav20]

take