OpenAI rate-limit metadata is always EmptyRateLimit in Spring AI 2.0.0

Bug description

When using the OpenAI chat model with Spring AI 2.0.0, the rate-limit metadata returned by ChatResponse is always an instance of EmptyRateLimit.

The completion, model metadata, and token usage are populated correctly, but the OpenAI rate-limit response headers do not appear to be mapped to ChatResponseMetadata.

I believe rate-limit metadata was available in earlier Spring AI 2.0.0 milestone versions, possibly up to 2.0.0-M1, but it is no longer available in the final 2.0.0 release.

Environment

Spring AI: 2.0.0
OpenAI chat model
Model: gpt-5-mini-2025-08-07
Java: 21

Steps to reproduce

ChatResponse chatResponse = chatClient.prompt()
        .user("Which country has the largest population in Europe?")
        .call()
        .chatResponse();

log.info("completion = {}", chatResponse.getResult().getOutput().getText());

log.info("model = {}", chatResponse.getMetadata().getModel());

var usage = chatResponse.getMetadata().getUsage();
log.info("usage = {}", usage);

var rateLimit = chatResponse.getMetadata().getRateLimit();
log.info("rateLimit = {}", rateLimit);

The completion, model, and usage information are returned correctly.

model = gpt-5-mini-2025-08-07
usage = DefaultUsage{
    promptTokens=20,
    completionTokens=655,
    totalTokens=675,
    cacheReadInputTokens=0
}
rateLimit = org.springframework.ai.chat.metadata.EmptyRateLimit@24eecabf

chatResponse.getMetadata().getRateLimit() always returns EmptyRateLimit.

Expected behavior

The OpenAI rate-limit response headers should be mapped to ChatResponseMetadata, allowing request and token limits, remaining values, and reset times to be accessed through getRateLimit().

Start by tracing how the ChatClient call populates ChatResponseMetadata, focusing on getRateLimit() and the OpenAI response-header mapping. Reproduce the issue with the provided snippet and verify that request and token limits, remaining values, and reset times are accessible through getRateLimit() instead of EmptyRateLimit.