For Reactor Netty and Reactor Netty 2 (with Netty 5), the attributes are
stored as an `Attribute` on the netty channel. Reactor Netty replaces
the channel with a static placeholder one once the request has been
processed and as such, capturing the native request in the test will
lead to asserting an attribute-less channel.
This change switches to capturing the `Attribute` itself, which is a
value-holder that can be asserted later on.
Closes gh-33909
d597d2e159