test: add unit and integration tests for linked-clone entrypoint lifecycle

Integration tests against a real SPDK target:
- Linked-clone volume read/write and independent snapshots
- Source replica/snapshot deletion blocked by active clone entrypoints
- Recovery of entrypoint and clone info after restart
- syncCloneEntrypoints auto-cleanup of childless entrypoints and
  orphaned tmp-head lvols (including correct SPDK state simulation
  via BdevLvolSnapshot to create a read-only ep + undeleted tmp-head)
- Repair of broken clone lineage via syncCloneReplicaInfo
- N-replica simultaneous linked-clone via DstReplicaSrcReplicaPairMap

Unit tests (no SPDK required):
- syncWithBdevLvolMap dispatch to correct sync path
- syncCloneReplicaInfo cases 1/2/3 covering entrypoint lookup logic
- Full Sync() execution paths (clone replica detection, error paths)

Also refactor Sync() to delegate to syncWithBdevLvolMap for testability;
spdkClient=nil skips validateAndUpdate so unit tests can exercise sync
logic without a live SPDK connection.

Longhorn 12552

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Shuo Wu <shuo.wu@suse.com>

Author: shuo-wu