In order to know the full database size we need to be able to sum b-tree sizes. In this change we'll aggregate the size of all the cookies in the b-tree and save that information to the checkpoint metadata for the tree.

To implement this:

1. Add a bytes_total field to the __wt_btree structure.
2. When a block is written via __wti_block_disagg_write_internal update the total size of the b-tree.
3. When a block is free'd / discarded decrease the total via: __wti_block_disagg_page_discard.
4. When a checkpoint is taken update the ckpt->size field, in meta_ckpt.c::__wt_meta_ckptlist_set.

Item 4 is effectively this patch:

        if (F_ISSET(ckpt, WT_CKPT_ADD)) {
            ckpt->next_page_id = btree->next_page_id;
            /* For disaggregated storage, save the current total compressed bytes to ckpt->size. */
            if (F_ISSET(btree, WT_BTREE_DISAGGREGATED)) {
                ckpt->size = __wt_atomic_load_uint64_relaxed(&btree->bytes_compressed_total);
            }
        } 

Write a unit test to validate that the b-tree size is being written out via checkpoint. 

Hint: This is test_disagg_checkpoint_size01.py on the PoC.

Scope:

1. Track b-tree level database size.
2. Write a test.