Improve diagnostics for missing ServletContext

[xoruddl]

Improves the diagnostics when Spring MVC resource handling is configured without a ServletContext.

The new failure analyzer detects a failed resourceHandlerMapping with a No ServletContext set cause and advises removing @EnableWebMvc from a test slice configuration or using @WebMvcTest.

Includes unit and integration tests for the analyzer.

Fixes #16936

[wilkinsona]

Thanks for the proposal. As the test demonstrates, the analysis will kick in even when the test's configuration isn't using @EnableWebMvc. This results in a misleading action for correcting the problem.

[xoruddl]

Thank you for the feedback. You are right that relying only on the exception message and bean name could result in misleading guidance.

I have updated the analyzer so that it is loaded from spring-boot-webmvc and only produces an analysis when an @EnableWebMvc configuration is actually present. The analysis now also identifies the configuration class or classes where @EnableWebMvc was found and provides guidance for choosing @SpringBootTest or @WebMvcTest when a web test configuration is required.

I have replaced the synthetic integration test with one that reproduces the failure using @EnableWebMvc. I have also added negative tests for a configuration without @EnableWebMvc, a failure involving a different bean, and a failure with a different message.

Thank you again for pointing this out. Please let me know if you think another approach would be preferable.