Add WebAPI middleware for multi-instance Roslyn Bridge routing

Add RoslynBridge.WebApi - ASP.NET Core 8.0 middleware that:
- Provides a centralized REST API for accessing multiple VS instances
- Manages instance registry with discovery by port, solution, or PID
- Proxies requests to the appropriate VS instance
- Tracks request/response history for debugging
- Auto-cleanup of stale instances via background service

Features:
- Health endpoints: /api/health, /api/health/ping
- Roslyn endpoints: /api/roslyn/projects, /api/roslyn/diagnostics, etc.
- Instance management: /api/instances (register, heartbeat, unregister)
- History tracking: /api/history, /api/history/stats
- Swagger UI at root (/) for API documentation
- CORS enabled for web applications

Services:
- InstanceRegistryService: Thread-safe registry of VS instances
- HistoryService: In-memory request/response history (max 1000 entries)
- InstanceCleanupService: Background service to remove stale instances
- RoslynBridgeClient: HTTP client for proxying to VS instances

Update RoslynBridge.sln to include RoslynBridge.WebApi project.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

RoslynBridge.WebApi/Models/RoslynQueryResponse.cs

@@ -0,0 +1,27 @@
+namespace RoslynBridge.WebApi.Models;
+
+/// <summary>
+/// Response model for Roslyn query operations
+/// </summary>
+public class RoslynQueryResponse
+{
+    /// <summary>
+    /// Indicates whether the operation was successful
+    /// </summary>
+    public bool Success { get; set; }
+
+    /// <summary>
+    /// Optional message providing additional context
+    /// </summary>
+    public string? Message { get; set; }
+
+    /// <summary>
+    /// The response data (structure varies by query type)
+    /// </summary>
+    public object? Data { get; set; }
+
+    /// <summary>
+    /// Error message if the operation failed
+    /// </summary>
+    public string? Error { get; set; }
+}