Binder context does not restore previous source causing missing data on Spring Boot 3.5 or above

[philwebb]

With the update to IndexedElementsBinder in Spring Boot 3.5 we now have the situation where Binder.Context.withSource(...) is called multiple times. This has revealed a long standing bug where the previous source was never restored.

Before Spring Boot 3.5, we got away with the problem because we only ever when one level deep when setting the source. With Spring Boot 3.5, we get a regression which causes some data to not be bound. I think this is the cause of #46038