Add comprehensive Copilot instructions for code style and Java 21 usage

[Copilot]

Establishes coding conventions and test patterns for GitHub Copilot to follow when generating code for this Java 21 project.

Changes

Java 21 Features

• Pattern matching for instanceof and switch expressions
• Records for data carriers
• Text blocks for multi-line strings
• Switch expressions over statements
• Sealed classes for closed hierarchies
• Virtual threads and sequenced collections

Code Style

• Method Design: Single return statements, short methods with clear single responsibility
• Immutability: Prefer immutable objects and collections (ImmutableSet, ImmutableMap)
• Keyword Usage: Use final sparsely, only where meaningful
• Clean Code: SOLID principles, composition over inheritance, clarity over cleverness

Test Patterns

• Structure: Arrange-Act-Assert with line comment sections
• Naming: Use sut for system under test
• Coverage: Test happy paths, edge cases, boundary conditions, and error handling

Example test structure:

@Test
void testMethodName() {
    // Arrange
    SomeClass sut = new SomeClass();
    
    // Act
    ResultType result = sut.methodUnderTest();
    
    // Assert
    assertEquals(expectedValue, result);
}

[!WARNING]

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title> <issue_description>use Java 21 features, whereever it makes sense.

Help me write clean, efficient, and maintainable code. Use the latest best practices and design patterns.

I prefer single-return methods. I prefer immutable objects. I prefer short methods that do one thing well. Use keyword final sparsely.

For unit tests use the arrange-act-assert pattern, use JUnit 5 and Mockito for mocking, use the name 'sut' for the system under test and ensure good test coverage. Insert line comments with Arrange, Act and Assert sections in the tests. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes tbsalling/aisutils#20

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.