When the filter is exposed as a bean (directly or through a
registration bean), it's picked up by the auto-configuration of
MockMvc. This causes a problem as MockMvc does not call init on a
filter before it's used and WebSocketUpgradeFilter fails with a
NullPointerException if its doFilter method is called when its init
method has not been called.
This commit reworks the WebSocket auto-configuration to use a
ServletContextInitalizer to register WebSocketUpgradeFilter rather
than a FilterRegistrationBean. This ensure that the filter is still
registered at the required position in the chain (last filter before
the servlet) while also preventing it from being registered with the
auto-configured MockMvc in tests.
Closes gh-37660
6353603d63