fix(GH-5922): Preserve OpenAI response metadata

[yaner-here]

Fixes #5922. Preserves unmapped root-level OpenAI chat completion response fields in ChatResponseMetadata.

The OpenAI SDK exposes provider-specific root fields through ChatCompletion._additionalProperties(). These fields were previously dropped when Spring AI mapped ChatCompletion into ChatResponseMetadata. This change copies those additional properties into the metadata map.

[ilayaperumalg]

Thanks for the PR! It's a great catch to preserve these additional properties.

However, there is one significant issue with leaking SDK-specific types into Spring AI's generic abstractions, and one minor refactoring suggestion:

1. Leaking com.openai.core.JsonValue:
   By doing result._additionalProperties().forEach(metadataBuilder::keyValue);, the map is populated with OpenAI SDK's JsonValue objects. As shown in the added test (metadata.<JsonValue>get(\non_null_field\)), this forces users to cast values to an OpenAI-specific class. This breaks Spring AI's portability—if a user switches to another provider, their code expecting JsonValue will break.

Recommendation: Unwrap the JsonValue into standard Java types (String, Number, Boolean, List, Map) before adding them. You can use Jackson for this:

result._additionalProperties().forEach((key, jsonValue) -> {
    try {
        Object unmarshalled = ModelOptionsUtils.JSON_MAPPER.readValue(jsonValue.toString(), Object.class);
        metadataBuilder.keyValue(key, unmarshalled);
    } catch (Exception e) {
        metadataBuilder.keyValue(key, jsonValue.toString());
    }
});

2. Minor Map Copying Simplification:
   In from(ChatResponseMetadata chatResponseMetadata, Usage usage), creating a temporary map via Streams is fine, but you could simplify it by iterating directly over the entries:

ChatResponseMetadata.Builder builder = ChatResponseMetadata.builder()
    .id(chatResponseMetadata.getId())
    .usage(usage)
    .model(chatResponseMetadata.getModel());

chatResponseMetadata.entrySet().forEach(e -> builder.keyValue(e.getKey(), e.getValue()));

return builder.build();

[yaner-here]

Thanks for the review! All suggestions have been addressed.

---

Slight implementation difference

There's a slight difference. OpenAI's JsonValue.toString() returns non-standard JSON string. For example, JsonValue.from(Map.of("key1", 1, "key2", 2)).toString() actually returns {key2=2, key1=1}, which is expected to be {"key2": 2, "key1": 1}, thus can't be handled by Jackson.

tools.jackson.core.exc.StreamReadException: Unrecognized token 'key2': was expecting (JSON String, Number, Array, Object or token 'null', 'true' or 'false')
 at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); byte offset: #UNKNOWN]
	at tools.jackson.core.JsonParser._constructReadException(JsonParser.java:1856)
	at tools.jackson.core.json.ReaderBasedJsonParser._reportInvalidToken(ReaderBasedJsonParser.java:3130)
	at tools.jackson.core.json.ReaderBasedJsonParser._handleOddValue(ReaderBasedJsonParser.java:2075)
	at tools.jackson.core.json.ReaderBasedJsonParser.nextToken(ReaderBasedJsonParser.java:809)
	at tools.jackson.databind.deser.jdk.UntypedObjectDeserializerNR._deserializeNR(UntypedObjectDeserializerNR.java:261)
	at tools.jackson.databind.deser.jdk.UntypedObjectDeserializerNR.deserialize(UntypedObjectDeserializerNR.java:65)
	at tools.jackson.databind.deser.DeserializationContextExt.readRootValue(DeserializationContextExt.java:266)
	at tools.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2639)

So I use ObjectMapper to get the valid JSON string.

---

Another better implementation?

BTW, JsonValue.convert(Object.class) could perfectly converting itself to Java types. For example, JsonValue.from(List.of("abc", 123, Map.of("key", "value"))).convert(Object.class) returns the expected nested instance.

Would JsonValue.convert(Object.class) be better? I would appreciate your thoughts on this.