RestClient readWithMessageConverters method hiding I/O Exceptions with incorrect error message.

[Nicolas-Miranda]

The org.springframework.web.client.DefaultRestClient#readWithMessageConverters method from spring-web:7.0.8 is hiding I/O Exceptions (SocketTimeouts from connectTimeout/readTimeout) as a RestClientException with an unrelated content-type incompatibility message:

throw new RestClientException("Error while extracting response for type [" + ResolvableType.forType(bodyType) + "] and content type [" + contentType + "]", cause);

For example a SocketTimeout exception triggered by a readTimeout set in a client configured with Accept/Content-Type headers for "application/json", throws a RestClientException with the message:

"Error while extracting response for type [MyServiceResponse] and content type [application/octet-stream]"

This message is misleading, causing confusion in the actual underlying issue. Ideally if the issue is I/O related, the readWithMessageConverters method should throw a ResourceAccessException with the proper message.

[fudianchn]

定位了根因并有一个向后兼容的修复方案（暂未提 PR，因本地 Gradle 构建受网络阻断无法验证；先贴方案供参考）。

根因：DefaultRestClient.readWithMessageConverters 的 catch 块把 UncheckedIOException | IOException | HttpMessageNotReadableException 统一包成 RestClientException("Error while extracting response for type [...] and content type [...]", cause)——即使 cause 是 I/O 异常（如 SocketTimeout），消息却指向 content type，产生误导。

修复：cause 为 IOException 时改抛 ResourceAccessException（I/O 专用消息）。这是向后兼容的，因为 ResourceAccessException extends RestClientException（子类），捕获 RestClientException 的调用方仍能捕获。而且该类里已有同样用 ResourceAccessException 表达 I/O 错误的先例（createResourceAccessException）。

catch (UncheckedIOException | IOException | HttpMessageNotReadableException exc) {
    Throwable cause = (exc instanceof UncheckedIOException u) ? u.getCause() : exc;
    if (cause instanceof IOException ioException) {
        throw new ResourceAccessException(
                "I/O error while extracting response for type [" + ResolvableType.forType(bodyType) + "]",
                ioException);
    }
    throw new RestClientException("Error while extracting response for type [" +
            ResolvableType.forType(bodyType) + "] and content type [" + contentType + "]", cause);
}

测试思路：mock ClientHttpResponse.getBody() 抛 IOException，retrieve().body(String.class) 应抛 ResourceAccessException（而非带 content-type 消息的 RestClientException）。