Respect IsCollapsable in proportional cleanup

[wieslawsoltes]

PR Summary: Issue 933 - Preserve Non-Collapsable ProportionalDock

Summary

This change fixes issue #933 where ProportionalDock.IsCollapsable = false prevented empty-dock collapsing but did not prevent single-child simplification. The proportional cleanup routine now respects IsCollapsable, so non-collapsable proportional docks remain in the layout tree even when left with a single child.

Problem

FactoryBase.CollapseDock correctly checks IsCollapsable for empty-dock removal. However, when a child dock is collapsed and the parent proportional dock ends up with a single child, CleanupProportionalDockTree used to simplify the tree unconditionally. This could remove a structural ProportionalDock even when IsCollapsable = false.

Fix Overview

• CleanupProportionalDockTree now returns early when dock.IsCollapsable is false.
• This keeps non-collapsable proportional docks in the tree during simplification passes.
• Existing behavior for collapsable docks is unchanged.

Implementation Details

Code Change

The guard lives in FactoryBase.CleanupProportionalDockTree:

private void CleanupProportionalDockTree(IProportionalDock dock)
{
    if (dock.VisibleDockables == null || dock.Owner is not IDock owner || dock.Owner is IRootDock)
        return;
    if (!dock.IsCollapsable)
        return;

    // existing single-child simplification logic...
}

Regression Test

A new headless test covers the scenario from issue #933 by:

1. Creating a proportional dock (mainArea) with IsCollapsable = false that contains a document, a splitter, and a collapsable bottom pane.
2. Collapsing the empty bottom pane.
3. Verifying that mainArea remains in the tree with the document as its only child.

Test location:

• tests/Dock.Avalonia.HeadlessTests/FactorySplitTests.cs

Behavior Changes

• Before: A ProportionalDock with IsCollapsable = false could still be simplified away when it had a single child.
• After: Non-collapsable proportional docks are preserved during cleanup, even when left with a single child.

Usage Guidance

If you want a proportional dock to stay as a persistent layout container, set IsCollapsable = false on it:

var mainArea = new ProportionalDock
{
    Id = "MainArea",
    IsCollapsable = false,
    VisibleDockables = factory.CreateList<IDockable>()
};

This ensures that both empty-dock collapse and single-child simplification will not remove the dock. For docks that should be simplified when reduced to a single child, keep the default (IsCollapsable = true).

Tests

• Added: CollapseDock_DoesNotSimplify_NonCollapsable_ProportionalDock
• Suite: Dock.Avalonia.HeadlessTests

Fixes #933