Fix flaky Reactor Netty attributes test - #37303

Summary

Fix the flaky reactorNettyAttributes() test by waiting for Reactor Netty to release the pooled connection before asserting that its request attributes have been cleared.

The response body can complete before connection cleanup finishes. Consequently, the test could inspect the channel while the attributes were still present.

The test now uses HttpClient.doOnDisconnected to observe the connection release and waits for that signal before performing the assertion. Production behavior remains unchanged.

Closes #36589

Testing

Targeted test:

./gradlew :spring-webflux:test --tests "org.springframework.web.reactive.function.client.WebClientIntegrationTests.reactorNettyAttributes"`