Update copilot-instructions with executable guidelines (#50067)

* Update copilot-instructions with executable guidelines

Added instructions for encapsulating code in standalone executables and including PR commentary.

* Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Apply suggestion from @T-Gro

* Update .github/copilot-instructions.md

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Bill Wagner <wiwagn@microsoft.com>

Author: 3 people

.github/copilot-instructions.md

@@ -44,6 +44,7 @@ For snippets >6 lines:
 1. All code should use the latest stable versions/features.
 1. Create examples in both C# and Visual Basic unless the article referencing the snippet resides in the in the `csharp`, `fsharp`, and `visual-basic` language folders.
 1. When you add code, use code comments sparingly because they don't get localized. You can use them to briefly clarify code-specific details (such as logic, parameters, or edge cases). Put any critical information and context in the markdown text of the referencing article.
+1. IMPORTANT: For created code, always try to encapsulate it in an standalone executable (e.g. `dotnet fsi myFile.fsx` or `dotnet run myFile.cs`, add the necessary boilerplate/imports/usings where needed, and execute it.). Run it, and for every code snippet, include a PR commentary checking each code sample and proving what it has produced - this can be diagnostics, standard output, or a result value. That standalone file is just for the purpose of verification within copilot's execution environment, the published docs snippet should remain a subset as you would normally write to maximize clarity.
 
 ## File Naming
 
@@ -69,4 +70,4 @@ When assigned an issue or directly given a task in GitHub:
 3. Check for build warnings in the OpenPublishing.Build status check
 4. If warnings exist:
    - Click "View Details" to open the build report
-   - Resolve any build warnings you introduced
+   - Resolve any build warnings you introduced