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/Services/ChatStreamUpdateBuilder.cs

@@ -0,0 +1,29 @@
+using Luna.Shared;
+using Microsoft.Extensions.AI;
+
+namespace Luna.Core.Services;
+
+public class ChatStreamUpdateBuilder : IChatStreamUpdateBuilder
+{
+    public ChatStreamUpdate FromAiContent(AIContent content, string messageId)
+        => content switch
+        {
+            TextReasoningContent reasoning
+                => new ChatStreamUpdate(ChatStreamUpdateType.Text, reasoning.Text, "assistant", false, messageId),
+            TextContent text
+                => new ChatStreamUpdate(ChatStreamUpdateType.Text, text.Text, "assistant", false, messageId),
+            ToolCallContent toolCall
+                => new ChatStreamUpdate(ChatStreamUpdateType.ToolCall, toolCall.ToString(), "assistant", false, messageId),
+            ToolResultContent toolResult
+                => new ChatStreamUpdate(ChatStreamUpdateType.ToolResult, toolResult.ToString(), "assistant", false, messageId),
+            UsageContent usage
+                => new ChatStreamUpdate(ChatStreamUpdateType.Usage, usage.ToString(), "system", false, messageId),
+            _ => throw new NotSupportedException($"Unsupported content type: {content.GetType().Name}")
+        };
+    
+    public ChatStreamUpdate FromString(string content, string messageId) 
+        => new(ChatStreamUpdateType.Text, content, "assistant", false, messageId);
+    
+    public ChatStreamUpdate CreateCompleteUpdate(string messageId) 
+        => new(ChatStreamUpdateType.Complete, null, null, true, messageId);
+}