http2: optimize neverIndex lookup with SafeSet

Replace array-based neverIndex lookup with SafeSet for O(1) lookup
performance. This improves buildNgHeaderString performance from
O(n) to O(1) per header, providing significant speedup in hot path
scenarios with many headers.

Author: Ayoub-Mabrouk

lib/internal/http2/util.js

@@ -761,7 +761,10 @@ function buildNgHeaderString(arrayOrMap,
 
   const singles = new SafeSet();
   const sensitiveHeaders = arrayOrMap[kSensitiveHeaders] || emptyArray;
-  const neverIndex = sensitiveHeaders.map((v) => v.toLowerCase());
+  const neverIndex = new SafeSet();
+  for (let i = 0; i < sensitiveHeaders.length; i++) {
+    neverIndex.add(sensitiveHeaders[i].toLowerCase());
+  }
 
   function processHeader(key, value) {
     key = key.toLowerCase();
@@ -787,7 +790,7 @@ function buildNgHeaderString(arrayOrMap,
         throw new ERR_HTTP2_HEADER_SINGLE_VALUE(key);
       singles.add(key);
     }
-    const flags = neverIndex.includes(key) ?
+    const flags = neverIndex.has(key) ?
       kNeverIndexFlag :
       kNoHeaderFlags;
     if (key[0] === ':') {