Fixes #5555. Preserve explicit Scheme role reassignment

[YourRobotOverlord]

• Fixes Copying a Scheme and reassigning an identical role removes its explicit value #5555

Description

Preserves an explicitly assigned Scheme role when a copied scheme
reassigns that role to the same Attribute.

Previously, SetAttributeForRoleProperty compared the assignment with the
resolved role value and converted a matching value to null. For copied
schemes, this erased the copied explicit value and caused the role to be
derived from Normal.

Tests

Added a regression test covering an identical Editable reassignment after
copying and changing Normal.

• Full parallelizable suite: 17,453 passed, 17 skipped
• Scheme tests: 46 passed
• git diff --check: passed

To pull down this PR locally:

git remote add copilot https://github.com/YourRobotOverlord/Terminal.Gui.git
git fetch copilot codex/fix-scheme-explicit-role-reassignment
git checkout copilot/codex/fix-scheme-explicit-role-reassignment

[Copilot]

Pull request overview

Fixes a Scheme copy/initializer edge case where reassigning an explicitly set role to the same Attribute value could incorrectly clear the explicit backing field and cause the role to be re-derived from Normal.

Changes:

• Update Scheme.SetAttributeForRoleProperty to keep an already-explicit role explicit when reassigned the same value.
• Add a regression test covering explicit Editable preservation after copying and changing Normal.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Terminal.Gui/Drawing/Scheme.cs	Preserves explicit role assignments when reassigned to the same value during initialization/copy scenarios.
Tests/UnitTestsParallelizable/Drawing/SchemeTests.cs	Adds regression coverage validating explicit Editable remains explicit after copy + reassignment.

[tig]

LGTM. Good catch.