Fix Jetty DataBufferFactory memory leak

Prior to this commit, gh-32097 added native support for Jetty for both
client and server integrations. The `JettyDataBufferFactory` was
promoted as a first class citizen, extracted from a private class in the
client support. To accomodate with server-side requirements, an extra
`buffer.retain()` call was performed.
While this is useful for server-side support, this introduced a bug in
the data buffer factory, as wrapping an existing chunk means that this
chunk is already retained.

This commit fixes the buffer factory implementation and moved existing
tests from mocks to actual pooled buffer implementations from Jetty.
The extra `buffer.retain()` is now done from the server support, right
before wrapping the buffer.

Fixes gh-35319

spring-web/src/main/java/org/springframework/http/server/reactive/JettyCoreServerHttpRequest.java

@@ -115,7 +115,10 @@ class JettyCoreServerHttpRequest extends AbstractServerHttpRequest {
 		// We access the request body as a Flow.Publisher, which is wrapped as an org.reactivestreams.Publisher and
 		// then wrapped as a Flux.
 		return Flux.from(FlowAdapters.toPublisher(Content.Source.asPublisher(this.request)))
-				.map(this.dataBufferFactory::wrap);
+				.map(chunk -> {
+					chunk.retain();
+					return this.dataBufferFactory.wrap(chunk);
+				});
 	}
 
 }