feat: Add IChatClient and IEmbeddingGenerator MEAI integration

Implement Microsoft.Extensions.AI interfaces (IChatClient, IEmbeddingGenerator)
for the OpenAI SDK, along with integration tests for chat completions,
tool calling, and embedding generation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: HavenDV

.gitignore

@@ -397,3 +397,7 @@ FodyWeavers.xsd
 # JetBrains Rider
 *.sln.iml
 /.idea/**
+
+# Environment files
+.env
+.env.*