Make objectType nullable in OpenAIChatCompletion.

Author: adamglin0

prompt/prompt-executor/prompt-executor-clients/prompt-executor-openai-client/src/commonMain/kotlin/ai/koog/prompt/executor/clients/openai/models/OpenAIChatCompletion.kt

@@ -227,7 +227,7 @@ public class OpenAIChatCompletionResponse(
     public val serviceTier: String? = null,
     public val systemFingerprint: String? = null,
     @SerialName("object")
-    public val objectType: String,
+    public val objectType: String? = null,
     public val usage: OpenAIUsage? = null,
 ) : OpenAIBaseLLMResponse