chore: import upstream snapshot with attribution

showcase/integrations/ms-agent-dotnet/agent/AimockHeaderMiddleware.cs

@@ -0,0 +1,37 @@
+// STOPGAP: This integration-level header propagation replaces once copilotkit-sdk-dotnet
+// ships (Microsoft contribution, ETA mid-2026). When that SDK lands, delete this code
+// and use the SDK's built-in header propagation.
+// See: https://www.notion.so/copilotkit/3543aa3818528150b6acc5b872ad7fe5
+
+// TODO(copilotkit-sdk-dotnet): migrate to SDK-level header propagation
+public class AimockHeaderMiddleware
+{
+    private readonly RequestDelegate _next;
+    private readonly ILogger<AimockHeaderMiddleware> _logger;
+
+    public AimockHeaderMiddleware(RequestDelegate next, ILogger<AimockHeaderMiddleware> logger)
+    {
+        _next = next;
+        _logger = logger;
+    }
+
+    public async Task InvokeAsync(HttpContext context)
+    {
+        var headers = context.Request.Headers
+            .Where(h => h.Key.StartsWith("x-", StringComparison.OrdinalIgnoreCase))
+            .ToDictionary(h => h.Key, h => h.Value.ToString());
+        // Stash on HttpContext.Items (NOT an AsyncLocal): the value must survive
+        // the AG-UI SSE-pump ExecutionContext boundary so the outbound-LLM policy
+        // can read it via IHttpContextAccessor at call time.
+        AimockHeaderContext.Set(context, headers);
+        // CVDIAG inbound breadcrumb: the x-* headers (incl. x-diag-run-id /
+        // x-diag-hops / x-aimock-context) have now been captured onto
+        // HttpContext.Items for this request.
+        CvDiag.LogInbound(_logger, "backend-ms-agent-dotnet", AimockHeaderContext.Get(context));
+        // No finally-wipe: the captured headers are request-scoped — they live on
+        // this request's HttpContext and die with it. Wiping them in a finally
+        // raced the still-pumping SSE response and could clear the value before
+        // the outbound LLM call read it.
+        await _next(context);
+    }
+}