fix(grid): correct calcPixelWidth for collapsed column groups to prevent header/cell misalignment on scroll

[Copilot]

When a collapsible column group is collapsed and child columns have explicit widths, horizontal scrolling causes standalone columns' headers and cells to misalign. The header virtual scroll uses calcPixelWidth from the column group object; the cell virtual scroll uses calcPixelWidth from individual leaf columns. On collapse, setExpandCollapseState() updates children's hidden states but never invalidates the group's cached calcPixelWidth, leaving it stale (full expanded width). The resulting size cache mismatch grows with scroll distance.

Changes

• IgxColumnGroupComponent (column-group.component.ts) — Override setExpandCollapseState() to call cacheCalcWidth() immediately after children's hidden states are mutated, so calcPixelWidth reflects only visible children before the next change detection cycle:

protected override setExpandCollapseState() {
    super.setExpandCollapseState();
    if (this.grid) {
        this.cacheCalcWidth();
    }
}

• grid-collapsible-columns.spec.ts — Regression test asserting that after initial collapse, the group's calcPixelWidth matches the dynamic width getter (sum of visible children only), not the full expanded width.
• grid-samples.spec.ts — New CollapsibleGroupInitiallyCollapsedComponent fixture with [expanded]="false" and explicit child column widths.

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