Fix uSES hydration in strict mode

Previously, we'd call and use getSnapshot on the second render resulting in `Warning: Text content did not match. Server: "Nay!" Client: "Yay!"` and then `Error: Text content does not match server-rendered HTML.`.

Fixes #26095. Closes #26113.

packages/react-reconciler/src/ReactFiberHooks.js

@@ -1799,15 +1799,28 @@ function updateSyncExternalStore<T>(
   // Read the current snapshot from the store on every render. This breaks the
   // normal rules of React, and only works because store updates are
   // always synchronous.
-  const nextSnapshot = getSnapshot();
-  if (__DEV__) {
-    if (!didWarnUncachedGetSnapshot) {
-      const cachedSnapshot = getSnapshot();
-      if (!is(nextSnapshot, cachedSnapshot)) {
-        console.error(
-          'The result of getSnapshot should be cached to avoid an infinite loop',
-        );
-        didWarnUncachedGetSnapshot = true;
+  let nextSnapshot;
+  const isHydrating = getIsHydrating();
+  if (isHydrating) {
+    // Needed for strict mode double render
+    if (getServerSnapshot === undefined) {
+      throw new Error(
+        'Missing getServerSnapshot, which is required for ' +
+          'server-rendered content. Will revert to client rendering.',
+      );
+    }
+    nextSnapshot = getServerSnapshot();
+  } else {
+    nextSnapshot = getSnapshot();
+    if (__DEV__) {
+      if (!didWarnUncachedGetSnapshot) {
+        const cachedSnapshot = getSnapshot();
+        if (!is(nextSnapshot, cachedSnapshot)) {
+          console.error(
+            'The result of getSnapshot should be cached to avoid an infinite loop',
+          );
+          didWarnUncachedGetSnapshot = true;
+        }
       }
     }
   }