fix(dashscope): preserve SSE error response body (#2278)

Fixes #2197.

DashScope SSE error events contain the model `request_id`, but
`DashScopeHttpClient.stream()` previously discarded the raw error
payload and constructed `DashScopeHttpException` with a `null` response
body.

This change preserves the parsed response together with its raw SSE
payload, then propagates that payload through
`DashScopeHttpException#getResponseBody()`. Callers can therefore
retrieve the DashScope `request_id` from the error response body.