lib: refactor lazy loading of undici for fetch method

Object.defineProperty is updated to lazily load the undici dependency for
the fetch method. This change allows for simpler and more reliable mocking
of the fetch method for testing purposes, resolving issues encountered
with premature method invocation during testing.

Fixes: #52015

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

@@ -59,27 +59,16 @@ installObjectURLMethods();
 
 {
   // https://fetch.spec.whatwg.org/#fetch-method
-  function set(value) {
-    ObjectDefineProperty(globalThis, 'fetch', {
-      __proto__: null,
-      writable: true,
-      value,
-    });
-  }
   ObjectDefineProperty(globalThis, 'fetch', {
     __proto__: null,
     configurable: true,
     enumerable: true,
-    set,
-    get() {
-      function fetch(input, init = undefined) {
-        // 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);
-      }
-      set(fetch);
-      return fetch;
+    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);
     },
   });
 }