Change the default leaf_page_max for newly created collections on disaggregated storage from WiredTiger's default of 32KB to 16KB. Attached storage should remain unchanged.

Collection tables are created without an explicit leaf_page_max, so they inherit WiredTiger's 32KB default. Index tables already set internal_page_max=16k,leaf_page_max=16k. This ticket is to make disaggregated storage use 16KB leaf pages for collections as per this decision doc.