Expose x-request-id in SSE streaming responses for Java SDK

Description

We need access to the x-request-id header to debug API requests with the OpenAI team, as described in the official docs:
👉 https://platform.openai.com/docs/api-reference/debugging-requests

Although x-request-id can be obtained when using:

OpenAIClientAsync.withRawResponse()

We are using SSE streaming in our project, below is example code:

OpenAIClientAsync currentClient = OpenAIOkHttpClientAsync.builder().build();

currentClient.responses().createStreaming(params)
    .subscribe(event -> {
        // business logic
    })
    .onCompleteFuture()
    .thenAccept(result -> {
        // handle completion
    })
    .exceptionally(ex -> {
        // handle error
        return null;
    });

Feature request

Please provide a way to access the x-request-id header in SSE streaming responses (e.g., via callback, metadata, or event property).
This would make debugging and support much easier without requiring a full switch to withRawResponse().

The issue names OpenAIClientAsync and responses().createStreaming, but no implementation files or tests. Start by tracing createStreaming through SSE response handling and compare it with withRawResponse() header access. Done means callers can reliably read x-request-id from streaming responses, with tests covering the exposed access path.