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/Controllers/HealthController.cs

@@ -0,0 +1,72 @@
+using Microsoft.AspNetCore.Mvc;
+using RoslynBridge.WebApi.Models;
+using RoslynBridge.WebApi.Services;
+
+namespace RoslynBridge.WebApi.Controllers;
+
+/// <summary>
+/// Health check and status controller
+/// </summary>
+[ApiController]
+[Route("api/[controller]")]
+[Produces("application/json")]
+public class HealthController : ControllerBase
+{
+    private readonly IRoslynBridgeClient _bridgeClient;
+    private readonly ILogger<HealthController> _logger;
+
+    public HealthController(IRoslynBridgeClient bridgeClient, ILogger<HealthController> logger)
+    {
+        _bridgeClient = bridgeClient;
+        _logger = logger;
+    }
+
+    /// <summary>
+    /// Get the health status of the middleware and Visual Studio plugin
+    /// </summary>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Health status information</returns>
+    /// <response code="200">Service is healthy</response>
+    /// <response code="503">Service is unhealthy</response>
+    [HttpGet]
+    [ProducesResponseType(typeof(HealthCheckResponse), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(HealthCheckResponse), StatusCodes.Status503ServiceUnavailable)]
+    public async Task<ActionResult<HealthCheckResponse>> GetHealth(CancellationToken cancellationToken)
+    {
+        var response = new HealthCheckResponse();
+
+        try
+        {
+            var isVsPluginHealthy = await _bridgeClient.IsHealthyAsync(null, cancellationToken);
+            response.VsPluginStatus = isVsPluginHealthy ? "Connected" : "Disconnected";
+
+            if (!isVsPluginHealthy)
+            {
+                response.Status = "Degraded";
+                _logger.LogWarning("Visual Studio plugin is not accessible");
+                return StatusCode(StatusCodes.Status503ServiceUnavailable, response);
+            }
+
+            _logger.LogInformation("Health check passed");
+            return Ok(response);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Health check failed");
+            response.Status = "Unhealthy";
+            response.VsPluginStatus = "Error";
+            return StatusCode(StatusCodes.Status503ServiceUnavailable, response);
+        }
+    }
+
+    /// <summary>
+    /// Simple ping endpoint
+    /// </summary>
+    /// <returns>Pong response</returns>
+    [HttpGet("ping")]
+    [ProducesResponseType(typeof(object), StatusCodes.Status200OK)]
+    public IActionResult Ping()
+    {
+        return Ok(new { message = "pong", timestamp = DateTime.UtcNow });
+    }
+}