GH-11235: Fix form data read in MultipartAware converter

[kdelay]

Fixes: gh-11235

HttpDslTests.testHttpProxyFlow() fails because of a regression in MultipartAwareFormHttpMessageConverter, not because of the reflective RestTestClient field path.

The converter used to delegate to an AllEncompassingFormHttpMessageConverter, which reads and writes both application/x-www-form-urlencoded and multipart/form-data. It now delegates to a MultipartHttpMessageConverter, which handles only multipart/form-data, so any form-urlencoded request body fails with:

org.springframework.http.converter.HttpMessageNotReadableException:
No multipart boundary found in Content-Type: "application/x-www-form-urlencoded"

The proxied request in that test is a POST to /service/internal with a form-urlencoded body, which is why it surfaced there. Any HTTP inbound gateway or channel adapter receiving a form post is affected, since HttpRequestHandlingEndpointSupport registers this converter first.

Changes:

• non-multipart reads and writes go to a FormHttpMessageConverter; the MultipartHttpMessageConverter is kept for multipart, selected by the same isMultipart() rule the old delegate used (a multipart content type, or a non-String value in the map)
• getSupportedMediaTypes() reports the media types of both delegates and canWrite() accepts both; with the multipart-only delegate, application/x-www-form-urlencoded had disappeared from both
• the read delegates against a MultiValueMap target type. HttpRequestHandlingEndpointSupport.extractRequestBody() falls back to byte[] when no request payload type is configured, and FormHttpMessageConverter.read() returns a single-value Map for a non-MultiValueMap type, which then fails the cast in this converter.
• setCharset() now applies to both delegates

Verification:

• MultipartAwareFormHttpMessageConverterTests is new; with only the test files applied and the converter reverted, all four of its tests fail, as does testHttpProxyFlow()
• ./gradlew :spring-integration-http:test — 235 tests, 0 failures
• ./gradlew :spring-integration-webflux:test — 35 tests, 0 failures
• checkstyleMain and checkstyleTest pass for the http module

[artembilan]

Duplicate of: #11236?

give you AI a hint to not fix if it is present on the issue already .