Add User-Agent header with 'spring-ai' value to OpenAI API client

- Adds 'User-Agent: spring-ai' header to all OpenAI API requests
- Maintains consistency with Azure OpenAI implementation
- Header is set in the RestClient default headers configuration
- Helps OpenAI identify requests coming from Spring AI for analytics and support

Author: SandraAhlgrimm

models/spring-ai-openai/src/main/java/org/springframework/ai/openai/api/OpenAiApi.java

@@ -132,6 +132,7 @@ public OpenAiApi(String baseUrl, ApiKey apiKey, MultiValueMap<String, String> he
 		// @formatter:off
 		Consumer<HttpHeaders> finalHeaders = h -> {
 			h.setContentType(MediaType.APPLICATION_JSON);
+			h.set("User-Agent", "spring-ai");
 			h.addAll(headers);
 		};
 		this.restClient = restClientBuilder.clone()