Document how to test WebClient code with a mock server [SPR-16482]

[spring-projects-issues]

Rossen Stoyanchev opened SPR-16482 and commented

While #19852 is unresolved, we could at least document the approach with using the OkHttp MockWebServer, so that's more accessible.

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Affects: 5.0.3

This issue is a sub-task of #19852

Referenced from: commits 3367df8