Add CanDockAsDocument to IDockable

[wieslawsoltes]

PR Summary: Add CanDockAsDocument

Summary

This change introduces CanDockAsDocument on IDockable to let dockables opt out of being docked as tabbed documents. The flag is enforced during docking validation and when invoking DockAsDocument, exposed in Avalonia UI bindings, and documented with tests to cover the expected behavior.

Implementation Details

• Added CanDockAsDocument to IDockable and all model base classes, with defaults set to true.
• Updated Avalonia JSON serializer property lists to include the new flag.
• Added a fluent setter WithCanDockAsDocument for easy configuration.
• Enforced the flag in DockManager validation so dockables that opt out cannot dock into IDocumentDock or IDocument targets.
• Guarded FactoryBase.DockAsDocument so programmatic docking respects the flag.
• Updated Fluent theme context menus and the diagnostics dockable debug view to surface the flag.

Key files:

• src/Dock.Model/Core/IDockable.cs
• src/Dock.Model/DockManager.cs
• src/Dock.Model/FactoryBase.Dockable.cs
• src/Dock.Model/FluentExtensions.cs
• src/Dock.Model.Avalonia/Core/DockableBase.cs
• src/Dock.Model.Avalonia/Json/AvaloniaDockSerializer.cs
• src/Dock.Avalonia.Themes.Fluent/Controls/ToolChromeControl.axaml
• src/Dock.Avalonia.Themes.Fluent/Controls/ToolPinItemControl.axaml
• src/Dock.Avalonia.Themes.Fluent/Controls/ToolTabStripItem.axaml
• src/Dock.Avalonia.Diagnostics/Controls/DockableDebugView.axaml

Usage

View model / code

var tool = new Tool
{
    Id = "Explorer",
    Title = "Explorer",
    CanDockAsDocument = false
};

Fluent configuration

tool.WithCanDockAsDocument(false);

Behavior notes

• When CanDockAsDocument is false, docking into IDocumentDock or IDocument targets is rejected.
• DockAsDocument does nothing for dockables with CanDockAsDocument = false.
• The “Dock as Tabbed Document” context menu item is hidden for dockables that opt out.

Tests

Covered by new/updated tests:

• tests/Dock.Avalonia.HeadlessTests/DockManagerTests.cs
• tests/Dock.Avalonia.HeadlessTests/DockManagerDocumentTests.cs
• tests/Dock.Avalonia.HeadlessTests/FactoryDockAsDocumentTests.cs
• tests/Dock.Model.UnitTests/DockGroupValidationTests.cs

Notes

• This flag is intended for docking-as-document scenarios only. Other docking rules (drag/drop, docking groups, pinning, floating) are unchanged.

Fixes #902