Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/chain-tests/src/multi_node/fork_recovery.rs`:
- Around line 340-365: The current if let (Some(first_old), Some(first_new)) =
(reorg_event.old_fork.first(), reorg_event.new_fork.first()) can silently skip
LCA assertions when either fork is empty; instead assert that both
reorg_event.old_fork and reorg_event.new_fork are non-empty up front (e.g.,
assert!(!reorg_event.old_fork.is_empty(), ...) and
assert!(!reorg_event.new_fork.is_empty(), ...)) to fail the test immediately if
find_reorg_split contract is violated, then unwrap or call .first().unwrap() to
bind first_old/first_new and proceed with the existing fork_parent checks
(referencing reorg_event, old_fork, new_fork, fork_parent, and
find_reorg_split).

In `@crates/chain-tests/src/multi_node/mempool_tests.rs`:
- Around line 968-993: The test currently uses a defensive `if let
(Some(first_old), Some(first_new)) = (reorg_event.old_fork.first(),
reorg_event.new_fork.first())` which can silently skip LCA checks if either fork
is empty; instead, assert that both forks are non-empty up front (e.g.
assert!(!reorg_event.old_fork.is_empty()) and
assert!(!reorg_event.new_fork.is_empty())), then safely obtain
first_old/first_new (via unwrap() or by calling first() after the asserts) and
perform the existing assertions comparing reorg_event.fork_parent.block_hash and
height to first_old.header().previous_block_hash / first_old.header().height and
similarly for first_new; this ensures violations of the find_reorg_split
contract fail the test rather than being ignored.