Fixes #4332 - Refactor test infrastructure to use modern ApplicationImpl.Coordinator instead of legacy MainLoop (#4335)

* Initial plan

* Refactor tests to use modern ApplicationImpl.Coordinator architecture instead of legacy Application.RunIteration

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Changes before error encountered

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Replace Application.RunIteration implementation to use modern ApplicationImpl.Coordinator architecture

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Simplify Application.RunIteration to directly delegate to ApplicationImpl.Coordinator without legacy infrastructure

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Update multitasking.md documentation to use Application.AddTimeout/RemoveTimeout instead of deprecated Application.MainLoop methods

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Refactor contribution guidelines and documentation

- Consolidated all contribution-related instructions into `CONTRIBUTING.md` as the single source of truth.
- Updated `copilot-instructions.md` and `AGENTS.md` to reference `CONTRIBUTING.md`.
- Enhanced build, test, and CI/CD workflow documentation.
- Updated repository structure documentation in `README.md` and `CONTRIBUTING.md`.
- Reinforced coding conventions and streamlined testing requirements.
- Documented GitHub Actions workflows and provided local CI replication steps.
- Refined API documentation requirements and PR guidelines.
- Simplified `.sln` file and removed redundant content.
- Updated links to point to the latest documentation resources.

* Revamp bug report template for clarity and detail

Updated the bug report template to improve structure, readability, and comprehensiveness:
- Specified "Terminal.Gui" in the `about` section.
- Replaced bold headings with Markdown heading syntax.
- Enhanced "To Reproduce" with placeholders for code and behavior details.
- Added an "Environment" section to collect OS, terminal, PowerShell, .NET, and `Terminal.Gui` version details.
- Expanded "Screenshots" to include GIFs and terminal output instructions.
- Removed outdated "Desktop" and "Smartphone" sections, consolidating relevant details.
- Improved "Additional Context" with prompts for consistency, prior behavior, and error messages.
- Streamlined "For Maintainers" instructions for setting project and milestone.

These changes aim to make bug reports more actionable and easier to reproduce.

* Remove [Obsolete] attribute and pragma warnings from Application.RunIteration - method now uses modern architecture internally

Co-authored-by: tig <585482+tig@users.noreply.github.com>

* Refactor Application.Run.cs for clarity and cleanup

Removed debug assertions and unused/commented-out code to simplify logic and improve maintainability. Renamed `forceDraw` to `forceRedraw` in `LayoutAndDraw` for better clarity. Removed the internal `OnNotifyStopRunState` method and its associated logic, indicating a refactor of the stop notification mechanism.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Tig <tig@users.noreply.github.com>
Co-authored-by: tig <585482+tig@users.noreply.github.com>

Tests/UnitTests/Views/TextViewTests.cs

@@ -4813,7 +4813,7 @@ This is the second line.
     public void Selected_Text_Shows ()
     {
         // Proves #3022 is fixed (TextField selected text does not show in v2)
-        var top = new Toplevel ();
+        Toplevel top = new ();
         top.Add (_textView);
         RunState rs = Application.Begin (top);
 
@@ -4833,7 +4833,7 @@ This is the second line.
 
         _textView.NewKeyDownEvent (Key.CursorRight.WithCtrl.WithShift);
 
-        Application.RunIteration (ref rs, true);
+        AutoInitShutdownAttribute.RunIteration ();
         Assert.Equal (new (4, 0), _textView.CursorPosition);
 
         //                                             TAB to jump between text fields.