ResourceUrlProvider detects wrong set of handler mappings

[bclozel]

See spring-projects/spring-boot#25113 for more context.

ResourceUrlProvider is listening on ContextRefreshedEvent to detect automatically instances of SimpleUrlHandlerMapping and use them for writing URLs to resources. Once handlers are detected the provider locks itself from further modifications.

With a parent/child context setup and under specific conditions due to ordering, the ResourceUrlProvider might receive a ContextRefreshedEvent from a child context first, thus detecting the wrong set of handlers and ignoring the expected ContextRefreshedEvent once all components are ready in the current context.

We must ensure that ResourceUrlProvider only considers events sent by the current context - this is a common pattern in other parts of Spring Framework.