⚡ perf: remove linq allocations in header filtering

global.json

@@ -1,5 +1,6 @@
 {
   "sdk": {
-    "version": "10.0.201"
+    "version": "10.0.103",
+    "rollForward": "latestFeature"
   }
 }

src/IdempotencyKey.AspNetCore/IdempotencyService.cs

@@ -159,7 +159,7 @@ public Task WriteValidationErrorAsync(HttpContext httpContext, string message)
         {
             if (httpContext.Request.Headers.TryGetValue(h, out var val))
             {
-                selectedHeaders[h] = val.Where(x => x != null).Select(x => x!).ToArray();
+                selectedHeaders[h] = FilterHeaderValues(val);
             }
         }
 
@@ -248,7 +248,7 @@ public async Task ProcessResponseAndCompleteAsync(HttpContext httpContext, Memor
         {
             if (!IsUnsafeHeader(h.Key))
             {
-                snapshot.Headers[h.Key] = h.Value.Where(x => x != null).Select(x => x!).ToArray();
+                snapshot.Headers[h.Key] = FilterHeaderValues(h.Value);
             }
         }
 
@@ -362,6 +362,42 @@ private static bool IsUnsafeHeader(string key)
         return !SafeReplayHeaders.Contains(key);
     }
 
+    private static string[] FilterHeaderValues(Microsoft.Extensions.Primitives.StringValues values)
+    {
+        var count = values.Count;
+        if (count == 0)
+        {
+            return Array.Empty<string>();
+        }
+
+        int nonNullCount = 0;
+        for (int i = 0; i < count; i++)
+        {
+            if (values[i] != null)
+            {
+                nonNullCount++;
+            }
+        }
+
+        if (nonNullCount == 0)
+        {
+            return Array.Empty<string>();
+        }
+
+        var result = new string[nonNullCount];
+        int index = 0;
+        for (int i = 0; i < count; i++)
+        {
+            var val = values[i];
+            if (val != null)
+            {
+                result[index++] = val;
+            }
+        }
+
+        return result;
+    }
+
     private Task WriteErrorResponseAsync(HttpContext httpContext, IdempotencyErrorContext error)
     {
         return _options.ErrorResponseWriter(httpContext, error);