Initial copilot-instructions.md with C# guidelines (#11328)

* Initial copilot-instructions.md with C# guidelines

* Update .github/copilot-instructions.md

Co-authored-by: Copilot <[email protected]>

* Update .github/copilot-instructions.md

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: Copilot <[email protected]>

Author: fabiocav

.github/copilot-instructions.md

@@ -0,0 +1,30 @@
+## General
+
+* Make only high-confidence suggestions when reviewing code changes.
+* Always use the latest version of C#, currently C# 13 features.
+* Never change global.json unless explicitly asked to.
+* Never change package.json or package-lock.json files unless explicitly asked to.
+* Never change NuGet.config files unless explicitly asked to.
+* For C# string comparisons, always use string.Equals with an appropriate, explicit, `StringComparison`.
+
+## Formatting
+
+* Apply code-formatting style defined in `.editorconfig`.
+* Prefer file-scoped namespace declarations and single-line using directives.
+* Insert a newline before the opening curly brace of any code block (e.g., after `if`, `for`, `while`, `foreach`, `using`, `try`, etc.).
+* Ensure that the final return statement of a method is on its own line.
+* Use pattern matching and switch expressions wherever possible.
+* Use `nameof` instead of string literals when referring to member names.
+* Ensure that XML doc comments are created for any public APIs. When applicable, include `<example>` and `<code>` documentation in the comments.
+
+### Nullable Reference Types
+
+* Declare variables non-nullable, and check for `null` at entry points.
+* Always use `is null` or `is not null` instead of `== null` or `!= null`.
+* Trust the C# null annotations and don't add null checks when the type system says a value cannot be null.
+
+### Testing
+
+* Do not emit "Act", "Arrange" or "Assert" comments.
+* Use Moq for mocking in tests.
+* Copy existing style in nearby files for test method names and capitalization.