Pinned dockable sizing fixes

[wieslawsoltes]

PR Summary

Overview

This change set fixes pinned dockable sizing regressions (shrink-on-toggle, size reset on re-open, incorrect initial size), prevents collapsed panel proportions from drifting, and adds tests to cover the new behavior.

Fixes and behavior changes

• Preserve pinned dockable size across overlay show/hide by reapplying stored pinned bounds and avoiding overwrites when sizes are already valid.
• Update pinned bounds only during splitter resize operations to stop layout passes from shrinking or resetting sizes.
• Skip pinned bounds tracking for dockables hosted inside ToolPinItemControl so pin item visuals do not overwrite the pinned dock size.
• Initialize pinned bounds from the current visible bounds (dockable first, then owner as fallback) when pinning, ensuring the initial pinned size reflects the active layout.
• Prevent ProportionalStackPanel from overwriting CollapsedProportion when any child is collapsed, avoiding incremental shrink on repeated pin/unpin cycles.

Tests added/updated

• ProportionalStackPanel: verify collapsed siblings do not reset stored collapsed proportions.
• Factory pin bounds: validate pinned bounds initialization uses dockable visible bounds and falls back to owner bounds.
• DockableControl tracking: assert pinned bounds update outside ToolPinItemControl and are ignored inside it.
• PinnedDockControl: ensure stored width is reapplied after layout changes and pinned bounds update on resize.

Files touched

• src/Dock.Controls.ProportionalStackPanel/Internal/ProportionManager.cs
• src/Dock.Model/FactoryBase.Dockable.cs
• src/Dock.Avalonia/Controls/PinnedDockControl.axaml.cs
• src/Dock.Avalonia/Controls/DockableControl.cs
• tests/Dock.Avalonia.UnitTests/Controls/ProportionalStackPanelTests.cs
• tests/Dock.Avalonia.HeadlessTests/FactoryPinBoundsTests.cs
• tests/Dock.Avalonia.HeadlessTests/DockableControlTrackingTests.cs
• tests/Dock.Avalonia.HeadlessTests/PinnedDockControlTests.cs

Test run

• dotnet test