Fix LinkedList.insert not updating the tail when inserting after the last node

[Darkslayer3324j]

LinkedList.insert(value, index) corrupts the list when the value is inserted right after the last node (index === length): the new node is linked after the old tail, but this.tail is not updated. The next append() then writes to the old tail's next, overwriting the inserted node:

const list = new LinkedList();
list.append(1).append(2);
list.insert(3, 2);
list.tail.value;   // 2, should be 3
list.append(4);
list.toString();   // '1,2,4', should be '1,2,3,4'   (3 is lost)

insert already handles an index beyond the end by appending and updating tail (the else branch), and index === 0 via prepend; only this in-between case (walking to the last node and linking after it) missed the tail update. The fix sets this.tail = newNode when the node was linked after the current tail.

Added a test for insert-then-append/deleteTail; it fails on master (1 failed) and passes with the change (21 passed). eslint runs clean via the pre-commit hook.

Found by differential fuzzing (random operation sequences on the data structures compared against a plain-array model); the rest of that pass (heaps, priority queue, hash table, Fenwick tree, disjoint set, LRU caches) matched the model.

Written with AI assistance (Claude Code); I reproduced the bug, traced the cause, and ran the tests and linter.