test/crimson/seastore: assert omap extent retirement on split/merge/balance

test/crimson/seastore: assert omap extent retirement on split/merge/balance

There was no coverage for extent leaks or double-frees during structural
omap B-tree transitions. While investigating a suspected extent leak, these
assertions were added and confirmed the caller-layer (parent-node) already
retires nodes correctly — no leak and no double-free.

Add count_live_omap_extents() to omap_manager_test_t, which scans the LBA
mapping space via scan_mapped_space() to count live OMAP_INNER/OMAP_LEAF
extents, and assert against it in leafnode/innernode_split_merge_balancing:
the live count returns to baseline after contraction, and is exactly 0 after
clearing the tree. This pins the caller-layer (parent-node) retirement
contract in CI.

Signed-off-by: Shai Fultheim <shai.fultheim@gmail.com>