Warn users that WebTestClient with MockMvc is not supported

[sjohnr]

WebTestClient + MockMvc is not currently supported. If possible, we should warn users with an IllegalStateException that this is not supported when the WebTestClient is set up. A note should also be added to the documentation.

Related gh-10841

[sjohnr]

See this comment for a possible workaround and note. If a workaround exists and works as intended, this issue could focus on updating documentation.