lib: implement lazy loading for fetch function

Instead of importing undici upfront, the module is now conditionally
required using require only when the fetch function is called for the
first time and the undici implementation is not already available. This
lazy loading approach improves resource usage and test reliability by
loading undici only when needed.

lib/internal/bootstrap/web/exposed-window-or-worker.js

@@ -57,17 +57,19 @@ defineReplaceableLazyAttribute(globalThis, 'perf_hooks', ['performance']);
 const { installObjectURLMethods } = require('internal/url');
 installObjectURLMethods();
 
+let fetchImpl;
 // https://fetch.spec.whatwg.org/#fetch-method
 ObjectDefineProperty(globalThis, 'fetch', {
   __proto__: null,
   configurable: true,
   enumerable: true,
   writable: true,
-  value: function fetch(input, init = undefined) { // eslint-disable-line func-name-matching
-    // Loading undici alone lead to promises which breaks lots of tests so we
-    // have to load it really lazily for now.
-    const { fetch: impl } = require('internal/deps/undici/undici');
-    return impl(input, init);
+  value: function value(input, init = undefined) {
+    if (!fetchImpl) { // Implement lazy loading of undici module for fetch function
+      const undiciModule = require('internal/deps/undici/undici');
+      fetchImpl = undiciModule.fetch;
+    }
+    return fetchImpl(input, init);
   },
 });