Context

PR 14913 changes how WiredTiger identifies shared trees: https://github.com/wiredtiger/wiredtiger/pull/14913

The follow-up comes from this review discussion: https://github.com/wiredtiger/wiredtiger/pull/14913#discussion_r4265693507

Today, shared-tree state is split between two objects. WT_BTREE_DISAGGREGATED identifies both live shared btrees and checkpoint views. WT_DHANDLE_SHARED_CKPT identifies only checkpoint-view dhandles. This works, but it divides one classification between WT_BTREE and WT_DATA_HANDLE and uses both the terms disaggregated and shared.

Proposed Change

Make WT_DATA_HANDLE the source of truth for runtime shared-tree classification. Add WT_DHANDLE_SHARED to every shared dhandle and keep WT_DHANDLE_SHARED_CKPT for checkpoint views. Replace runtime uses of WT_BTREE_DISAGGREGATED with these dhandle flags and clearly named helper functions.

The state model is:

• Local tree: both flags are clear.
• Live shared tree: only WT_DHANDLE_SHARED is set.
• Shared checkpoint view: both flags are set.

Set these flags when the dhandle configuration is established from block_manager=disagg or the legacy .wt_stable fallback. Runtime and hot-path code must use the dhandle state without performing a metadata lookup. Callers without a dhandle must continue to use an explicit URI/config classifier. The legacy name fallback remains until WT-18866 removes it.

Acceptance Criteria

• Every open shared btree has WT_DHANDLE_SHARED set, and WT_DHANDLE_SHARED_CKPT always implies WT_DHANDLE_SHARED.
• Live shared trees and shared checkpoint views can be distinguished using only dhandle state.
• WT_BTREE_DISAGGREGATED is removed as a separate source of shared-tree state, or any remaining uses are justified.
• Tests cover local, live shared, shared checkpoint, config-identified shared, and legacy .wt_stable trees.