Fix parent folder checkbox not auto-checked when all children are marked as viewed

[Copilot]

With manageCheckboxStateManually: true (added in #8527), VS Code doesn't apply checkboxState changes returned from getTreeItem() during refresh(). Directory checkbox states were only computed lazily in getTreeItem(), so parent folders never visually updated when all children were individually checked.

Changes

• DirectoryTreeNode: Made setCheckboxState public, added updateCheckboxFromChildren() to eagerly recompute state from children
• treeUtils.ts: After updating file node states in processCheckboxUpdates, walk up from each affected node and eagerly set ancestor directory checkbox states before firing the tree refresh

// Eagerly update ancestor directory checkbox states
const allAffected = [...checkedNodes, ...uncheckedNodes];
for (const node of allAffected) {
    let parent = node.getParent();
    while (parent instanceof DirectoryTreeNode) {
        parent.updateCheckboxFromChildren();
        parent = parent.getParent();
    }
}

• Tests: Added tests for allChildrenViewed() and updateCheckboxFromChildren() covering flat, nested, mixed, and state-transition scenarios

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