fix(data-structures): deep-copy nodes in BinarySearchTree/RedBlackTree.from()

[LeSingh1]

Problem

BinarySearchTree.from() and RedBlackTree.from() are documented to create an
independent copy of an existing tree, but they only performed a partial copy.
The traversal that walks the source tree created a copy of each child node and
fixed up the child's parent pointer, but it never re-linked the parent copy's
left/right pointers to point at those new child copies. Because
BinarySearchNode.from() / RedBlackNode.from() seed a copy's left/right
with the original node's children, the returned tree ended up still
referencing the source tree's descendant nodes.

RedBlackTree.from() was additionally worse: it seeded the result with the
source tree's root node itself (via setRoot(result, root)) rather than a
copy of it.

The net effect is that the "copy" shares nodes with the original, so mutating
the copy corrupts the original (and vice versa):

import { BinarySearchTree } from "@std/data-structures";

const original = BinarySearchTree.from([3, 10, 13, 4, 6, 7, 1, 14]);
const copy = BinarySearchTree.from(original);

copy.remove(7);
original.find(7); // null  ❌ — 7 was removed from the original too

Fixes #6314
Fixes #6315

Fix

• Re-link node.left / node.right to the freshly created child copies during
  the copy traversal in both BinarySearchTree.from() and
  RedBlackTree.from().
• In RedBlackTree.from(), copy the root node before seeding the result with it
  (so the traversal no longer mutates the source root).

The mapped / custom-comparator paths are unchanged (they already rebuild the
tree via insert()).

Verification

• Added regression tests for both trees asserting that a removal in either the
  copy or the original leaves the other intact. They fail on main
  (original.find(7) returns null) and pass with this change.
• deno test data_structures/ → 350 passed, 0 failed.
• deno fmt --check and deno lint on the changed files are clean.
• deno check on the changed files passes.