Add Luna.Core server with SignalR hub, session management, and tool system

Implement the core application server as an ASP.NET web host with:
- SignalR ChatHub for real-time client communication
- Session management with token estimation for context windowing
- Extensible tool system with attribute-based tool discovery
- Chat stream update builder for streaming AI responses
- App configuration with appsettings for development and production

Luna.Core/Tools/ToolsProvider.cs

@@ -0,0 +1,9 @@
+using Microsoft.Extensions.AI;
+
+namespace Luna.Core.Tools;
+
+public sealed class ToolsProvider(IEnumerable<IToolbox> toolboxes) : IToolsProvider
+{
+    public IEnumerable<AITool> GetTools()
+        => toolboxes.SelectMany(t => t.GetTools());
+}