Enhance GitHub Copilot instructions with comprehensive validation and accurate documentation

[Copilot]

This PR significantly enhances the existing .github/copilot-instructions.md file with validated commands, accurate timing estimates, and comprehensive testing scenarios to help GitHub Copilot agents work effectively in the xUnit Dependency Injection codebase.

Key Improvements

Validated All Commands: Every command in the instructions has been tested and verified to work correctly with .NET 9.0 SDK (9.0.304). All build times and expectations have been measured and documented accurately.

Updated Test Counts: Corrected the test count from 9 to 43 tests to reflect the current state of the test suite, ensuring agents have accurate expectations.

Enhanced Manual Testing: Added specific test filter commands for validating individual features:

• Property injection testing with dotnet test --filter "TestCalculatorThroughPropertyInjection"
• Keyed services validation with dotnet test --filter "GetKeyedService"
• Factory pattern testing with dotnet test --filter "TestConstructorInjectionViaFactory"
• Configuration testing with dotnet test --filter "TestSecretValues"

Comprehensive Test Pattern Documentation: Added detailed examples of all three dependency injection approaches:

1. Traditional fixture-based approach using TestBed<TFixture>
2. Property injection approach using TestBedWithDI<TFixture> with [Inject] attributes
3. Experimental factory pattern for true constructor injection

Improved Troubleshooting: Expanded the troubleshooting section with common validation failures and their solutions, based on actual testing experience.

Accurate Performance Expectations: All timing estimates have been measured and validated:

• Package restore: ~8 seconds
• Build: ~6 seconds
• Test execution: ~11 seconds (43 tests)
• Code formatting: ~8 seconds
• Package creation: ~1.5 seconds

The instructions now provide a complete, accurate reference that enables any GitHub Copilot agent to immediately understand the project structure, build process, testing patterns, and validation procedures without needing additional exploration.

Fixes #318.

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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.