Bind multipart files in ProxyingHandlerMethodArgumentResolver.

[jmsmg]

Interface-based form-backing objects created by ProxyingHandlerMethodArgumentResolver return null for MultipartFile properties, see #1136.

Cause. createAttribute binds request.getParameterMap() only. The files of a multipart request live in MultipartRequest#getMultiFileMap(), so they never reach the Map backing the projection proxy, and bindRequestParameters is a no-op, so there is no second binding pass either. In a @NullMarked context the proxy's nullness validation turns the null into a NullPointerException ("Return value is null but must not be null").

Change. createAttribute now also adds the files of a MultipartRequest to the property values, following WebDataBinder#bindMultipart: a single file is bound as is, multiple files for the same name as a List<MultipartFile>. Requests without a MultipartRequest are unaffected.

Tests. Two unit tests in ProxyingHandlerMethodArgumentResolverUnitTests (bindsMultipartFileIntoProjectedPayload, bindsMultipleMultipartFilesIntoProjectedPayload) resolve a @ProjectedPayload interface from a MockMultipartHttpServletRequest. Both fail without the change and pass with it; EnableSpringDataWebSupportIntegrationTests still passes.

Open question. I kept the change local to the resolver. Alternatively MapDataBinder could extend WebRequestDataBinder and use bind(WebRequest), which would additionally bind Servlet Parts when no MultipartResolver is configured. Happy to switch if you prefer that route.

Closes #1136

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