feat: Add tests

[kaspermarstal]

• Reorganize test project with Unit/ and Integration/ folders
• Add unit tests for ArgumentParser, StructuredOutput, and CacheBehavior
• Add Excel integration tests using Ollama (gemma3:4b-it-qat)
• Add ExcelTestHelper with CalculateFull() for async function testing
• Fix xunit.runner.visualstudio version (2.8.2) for ExcelDna.Testing compatibility

Infrastructure improvements:

• Add environment-based configuration (DOTNET_ENVIRONMENT)
• Create appsettings.Testing.json with EnableCache=false for tests
• Add ModuleInitializer to set test environment before add-in loads
• Use unique cell ranges per test to avoid ExcelDNA async cache collisions

Bug fixes:

• Fix AdditionalPropertiesBehavior null check (check provider config, not prompt)
• Add stream:false to Ollama configuration to prevent hanging requests