Migrate ANNIvfPartitionExec to native DataFusion partitions

ANNIvfPartitionExec outputs a single partition and manages parallelism internally via tokio::spawn per delta index and spawn_cpu for partition searches. It should be migrated to output N partitions (where N = target_partitions).

Each partition should pull nprobe work from a shared queue rather than using a static upfront split, to handle uneven per-partition search costs and avoid stragglers. The existing early-stop AtomicUsize coordination (num_results_found) is acceptable small shared state — whichever partition updates the counter can stop new work from being pulled from the queue, regardless of which partition would have pulled next.

A TopK merge node is needed above ANNIvfPartitionExec to combine per-partition results. This can either be a new custom exec or use SortPreservingMergeExec + GlobalLimitExec if those give correct distance-ranked top-K semantics.

The tokio::spawn-per-delta fan-out should be replaced by partition-level execution.

Depends on #7081 (EnforceDistribution) to handle exchange nodes automatically.
Part of the migration tracked in #6967.