✨ Adds project guidelines and conventions for Github Copilot

Introduces detailed instructions for ASP.NET API, C#, Docker, MSBuild,
testing, and library creation. These guidelines aim to standardize
development practices and improve code quality across the project.

.github/instructions/testing.instructions.md

@@ -0,0 +1,23 @@
+---
+description: "Use when creating or modifying test projects under the tests/ folder. Covers naming conventions and structural guardrails."
+applyTo: "tests/**"
+---
+# Test Project Conventions
+
+Build configuration is handled by `tests/Directory.Build.props` — do not restate its settings.
+
+## Framework
+
+- **Testing**: TUnit.
+- **Mocking**: NSubstitute (analyzers auto-added when referenced).
+
+## Naming
+
+- Test project: `<ProjectUnderTest>.Tests` (matches InternalsVisibleTo default).
+- Test classes: `<ClassUnderTest>Tests`.
+- Test methods: descriptive names — `MethodName_Condition_ExpectedResult` or similar.
+
+## Structure
+
+- Follow **Arrange / Act / Assert**.
+- Always propose a set of basic tests for new or changed code.