Set 500 status for disconnected client exceptions

[king-407]

Summary

This PR updates Spring MVC disconnected-client exception handling to set the response status to 500 instead of leaving it at the default 200 status.

The change covers both paths discussed in gh-34481:

• DefaultHandlerExceptionResolver, which handles default Spring MVC exception resolution
• ExceptionHandlerExceptionResolver, where an @ExceptionHandler rethrows a disconnected-client style exception

For an actual disconnected client, the client will not observe the response. For cases where the exception is incorrectly classified as a disconnected-client exception, this avoids silently reporting a failed request as successful.

This is based on the latest issue discussion; happy to adjust the scope or implementation if the maintainers prefer a different approach.

Testing

• ./gradlew :spring-webmvc:test --tests org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolverTests --tests org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolverTests
• git diff --check

See gh-34481

[king-407]

This PR is based on the latest discussion around setting a 500 response status for disconnected-client exceptions instead of leaving the response at the default 200.

The change covers both Spring MVC paths discussed in the issue: DefaultHandlerExceptionResolver and ExceptionHandlerExceptionResolver. I also added focused tests for IOException("Broken pipe"), including the case where an @ExceptionHandler rethrows the exception.

Happy to adjust the scope or implementation if the maintainers prefer a different approach.