Fixes #5509 branch color getter invoker now applies Active attribute

[YourRobotOverlord]

Fixes

• Fixes Using custom ColorGetter breaks unfocused Active attribute on TreeView Branch #5509

Proposed Changes/Todos

• Extends Tree View scenario to include a custom color delegate. Enable/disable using new Style menu.
• Fixes delegate invoker to also apply Active attribute.

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

Before

After

[Copilot]

Pull request overview

Fixes TreeView branch rendering when a custom ColorGetter is used by ensuring the unfocused-but-selected state uses the Active attribute, and updates the UICatalog scenario/docs to demonstrate and explain the behavior.

Changes:

• Update TreeView branch draw logic to use Scheme.Active for selected branches when the TreeView is unfocused.
• Expand TreeView<T>.ColorGetter XML docs to clarify which Scheme roles are consumed.
• Extend the UICatalog TreeView scenario with a toggleable custom color delegate.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Terminal.Gui/Views/TreeView/TreeViewT.cs	Improves ColorGetter documentation, including how Normal/Focus/Active are used.
Terminal.Gui/Views/TreeView/Branch.cs	Fixes selection coloring with ColorGetter by applying Active when the TreeView lacks focus.
Examples/UICatalog/Scenarios/TreeUseCases.cs	Adds a scenario toggle to demonstrate ColorGetter-based per-node coloring.

Comments suppressed due to low confidence (1)

Terminal.Gui/Views/TreeView/Branch.cs:219

• When a ColorGetter is set but returns null for a given model, the code currently sets modelColor = new Attribute () (white/black). This contradicts the ColorGetter XML docs ("null to use the default tree view scheme") and causes theme-inconsistent colors for nodes that intentionally fall back to the default scheme. Keep the existing modelColor (initialized from textColor) when modelScheme is null.

                                 : modelScheme.Normal;
            }
            else
            {
                modelColor = new Attribute ();