implement SearchUsers use case

backend/tests/Fakes/FakeUserRepository.php

@@ -63,6 +63,36 @@ class FakeUserRepository implements UserRepository
         return null;
     }
 
+    /**
+     * @return User[]
+     */
+    public function search(string $query): array
+    {
+        $needle = strtolower($query);
+        $results = [];
+        foreach ($this->existingUsers as $user) {
+            $displayName = strtolower($user->getDisplayName());
+            $email = strtolower($user->getEmail()->value());
+            if (
+                str_starts_with($displayName, $needle)
+                || str_starts_with($email, $needle)
+            ) {
+                $results[] = $this->copy($user);
+            }
+        }
+        usort(
+            $results,
+            function (User $left, User $right) {
+                return strcmp(
+                    $left->getDisplayName(),
+                    $right->getDisplayName(),
+                );
+            },
+        );
+
+        return $results;
+    }
+
     /**
      * @throws RuntimeException
      */