Migrate LanceScanExec to native DataFusion partitions

LanceScanExec outputs a single partition and manages I/O parallelism internally via try_buffered. It should be migrated to output N partitions (where N = target_partitions from the session config).

Rather than statically splitting fragments across partitions upfront, each partition should pull from a shared work queue of fragment reads. This avoids stragglers from uneven fragment sizes — a partition that finishes a small fragment quickly picks up the next available one rather than sitting idle.

This follows the same shared-queue pattern already used by FilteredReadExec in MultiplePartitions mode (Arc<AsyncMutex<task_stream>>).

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