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/Services/InstanceCleanupService.cs

@@ -0,0 +1,50 @@
+namespace RoslynBridge.WebApi.Services;
+
+/// <summary>
+/// Background service that periodically removes stale VS instances
+/// </summary>
+public class InstanceCleanupService : BackgroundService
+{
+    private readonly IServiceProvider _serviceProvider;
+    private readonly ILogger<InstanceCleanupService> _logger;
+    private readonly TimeSpan _cleanupInterval = TimeSpan.FromMinutes(1);
+    private readonly TimeSpan _staleTimeout = TimeSpan.FromMinutes(5);
+
+    public InstanceCleanupService(
+        IServiceProvider serviceProvider,
+        ILogger<InstanceCleanupService> logger)
+    {
+        _serviceProvider = serviceProvider;
+        _logger = logger;
+    }
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        _logger.LogInformation("Instance cleanup service started");
+
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            try
+            {
+                await Task.Delay(_cleanupInterval, stoppingToken);
+
+                // Get the registry service from scope
+                using var scope = _serviceProvider.CreateScope();
+                var registryService = scope.ServiceProvider.GetRequiredService<IInstanceRegistryService>();
+
+                registryService.RemoveStaleInstances(_staleTimeout);
+            }
+            catch (OperationCanceledException)
+            {
+                // Expected when stopping
+                break;
+            }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex, "Error during instance cleanup");
+            }
+        }
+
+        _logger.LogInformation("Instance cleanup service stopped");
+    }
+}