feat(rate-limiting): Implement core rate limiting functionality with configuration, decision-making, metrics, middleware, and service registration

- Add RateLimitConfig for configuration management with YAML binding support.
- Introduce RateLimitDecision to encapsulate the result of rate limit checks.
- Implement RateLimitMetrics for OpenTelemetry metrics tracking.
- Create RateLimitMiddleware for enforcing rate limits on incoming requests.
- Develop RateLimitService to orchestrate instance and environment rate limit checks.
- Add RateLimitServiceCollectionExtensions for dependency injection registration.

src/__Libraries/StellaOps.Router.Gateway/ApplicationBuilderExtensions.cs

@@ -1,5 +1,6 @@
 using StellaOps.Router.Gateway.Middleware;
 using StellaOps.Router.Gateway.OpenApi;
+using StellaOps.Router.Gateway.RateLimit;
 
 namespace StellaOps.Router.Gateway;
 
@@ -18,6 +19,9 @@ public static class ApplicationBuilderExtensions
         // Enforce payload limits first
         app.UseMiddleware<PayloadLimitsMiddleware>();
 
+        // Rate limiting (Sprint 1200_001_001)
+        app.UseRateLimiting();
+
         // Resolve endpoints from routing state
         app.UseMiddleware<EndpointResolutionMiddleware>();
 
@@ -30,6 +34,24 @@ public static class ApplicationBuilderExtensions
         return app;
     }
 
+    /// <summary>
+    /// Adds rate limiting middleware to the pipeline.
+    /// Sprint: SPRINT_1200_001_001_router_rate_limiting_core
+    /// Task: 1.6 - Wire into Router Pipeline
+    /// </summary>
+    /// <param name="app">The application builder.</param>
+    /// <returns>The application builder for chaining.</returns>
+    public static IApplicationBuilder UseRateLimiting(this IApplicationBuilder app)
+    {
+        // Only add if rate limit service is registered
+        var rateLimitService = app.ApplicationServices.GetService<RateLimitService>();
+        if (rateLimitService is not null)
+        {
+            app.UseMiddleware<RateLimitMiddleware>();
+        }
+        return app;
+    }
+
     /// <summary>
     /// Adds the router gateway middleware pipeline without payload limiting.
     /// </summary>
@@ -37,6 +59,9 @@ public static class ApplicationBuilderExtensions
     /// <returns>The application builder for chaining.</returns>
     public static IApplicationBuilder UseRouterGatewayCore(this IApplicationBuilder app)
     {
+        // Rate limiting (Sprint 1200_001_001)
+        app.UseRateLimiting();
+
         // Resolve endpoints from routing state
         app.UseMiddleware<EndpointResolutionMiddleware>();