✨ Updates API, C#, Docker, MSBuild, and Testing guidelines

Refines conventions for ASP.NET API endpoints, C# documentation,
Docker build arguments, MSBuild package management, and testing
method naming and structure. These changes aim to enhance clarity
and enforce best practices across the codebase.

.github/instructions/testing.instructions.md

@@ -15,9 +15,10 @@ Build configuration is handled by `tests/Directory.Build.props` — do not resta
 
 - Test project: `<ProjectUnderTest>.Tests` (matches InternalsVisibleTo default).
 - Test classes: `<ClassUnderTest>Tests`.
-- Test methods: descriptive names — `MethodName_Condition_ExpectedResult` or similar.
+- Test methods: must use `MethodName_Condition_ExpectedResult` format. Example: `Add_NegativeNumbers_ThrowsArgumentException`.
 
 ## Structure
 
 - Follow **Arrange / Act / Assert**.
-- Always propose a set of basic tests for new or changed code.
+- Always propose a set of basic tests for new or changed code. A basic test set must include at minimum: one happy-path test, one test per documented edge case or boundary condition, and one test for each expected exception.
+- If no edge cases are explicitly documented, use judgment to identify and cover the most likely boundary conditions (e.g. null inputs, empty collections, zero/max values) and note the assumption in a comment.