Added #include "firebase/storage/list_result.h" to storage_reference.h and storage_reference_desktop.h to resolve compilation errors related to undeclared identifier 'ListResult'.

Changed FIREBASE_ASSERT_RETURN to use is_valid() instead of
internal_->is_valid() in StorageReference::ListAll() and
StorageReference::List() methods.

This corrects the compilation error where 'is_valid' was not a member
of the internal PIMPL class. The assertion now correctly checks the
validity of the public StorageReference object before proceeding.

Changed the include from 'storage_internal_desktop.h' to
'storage_desktop.h' in list_result_desktop.h.

'storage_internal_desktop.h' does not exist; 'storage_desktop.h'
contains the definition for internal::StorageInternal, which is used by
ListResultInternal. This resolves the 'file not found' compilation error.