Fix memory reservation starvation in sort-merge

[xudong963]

Which issue does this PR close?

• Closes #.

Rationale for this change

This PR fixes memory reservation starvation in sort-merge when multiple sort partitions share a GreedyMemoryPool.

When multiple ExternalSorter instances run concurrently and share a single memory pool, the merge phase starves:

1. Each partition pre-reserves sort_spill_reservation_bytes via merge_reservation
2. When entering the merge phase, new_empty() was used to create a new reservation starting at 0 bytes, while the pre-reserved bytes sat idle in ExternalSorter.merge_reservation
3. Those freed bytes were immediately consumed by other partitions racing for memory
4. The merge could no longer allocate memory from the pool → OOM / starvation

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?