Fix for #967: Resolve ResponsesModel union to plain model name in URL path

`PrepareRequest.modelNameOrNull()` called `toString()` on whatever `model()`
returned, so a `ResponsesModel` leaked its debug form into the Azure deployment
path (`ResponsesModel{string=gpt-5.4}` instead of `gpt-5.4`). Resolve the union
through its visitor and add tests for the string, chat, and responses-only
variants.

Author: lzandman

openai-java-core/src/main/kotlin/com/openai/core/PrepareRequest.kt

@@ -5,6 +5,8 @@ package com.openai.core
 import com.openai.azure.addPathSegmentsForAzure
 import com.openai.azure.replaceBearerTokenForAzure
 import com.openai.core.http.HttpRequest
+import com.openai.models.ChatModel
+import com.openai.models.ResponsesModel
 import java.util.Optional
 import java.util.concurrent.CompletableFuture
 import kotlin.reflect.full.declaredFunctions
@@ -49,8 +51,29 @@ internal fun Params.modelNameOrNull(): String? {
             null
         }
 
-    return when (modelName) {
-        is Optional<*> -> modelName.orElse(null)?.toString()
-        else -> modelName?.toString()
+    val unwrappedModelName =
+        when (modelName) {
+            is Optional<*> -> modelName.orElse(null)
+            else -> modelName
+        }
+
+    return when (unwrappedModelName) {
+        // The `toString()` of a union type is a debug representation, so resolve its variant.
+        is ResponsesModel -> unwrappedModelName.modelNameOrNull()
+        else -> unwrappedModelName?.toString()
     }
 }
+
+private fun ResponsesModel.modelNameOrNull(): String? =
+    accept(
+        object : ResponsesModel.Visitor<String?> {
+            override fun visitString(string: String): String = string
+
+            override fun visitChat(chat: ChatModel): String = chat.toString()
+
+            override fun visitOnly(only: ResponsesModel.ResponsesOnlyModel): String =
+                only.toString()
+
+            override fun unknown(json: JsonValue?): String? = null
+        }
+    )

openai-java-core/src/test/kotlin/com/openai/core/PrepareRequestTest.kt

@@ -2,9 +2,12 @@ package com.openai.core
 
 import com.openai.core.http.Headers
 import com.openai.core.http.QueryParams
+import com.openai.models.ChatModel
+import com.openai.models.ResponsesModel
 import com.openai.models.chat.completions.ChatCompletionListParams
 import com.openai.models.embeddings.EmbeddingCreateParams
 import com.openai.models.embeddings.EmbeddingModel
+import com.openai.models.responses.ResponseCreateParams
 import com.openai.models.uploads.UploadCancelParams
 import org.assertj.core.api.Assertions.assertThat
 import org.assertj.core.api.Assertions.assertThatNoException
@@ -106,4 +109,30 @@ internal class PrepareRequestTest {
         assertThatNoException().isThrownBy { params.modelNameOrNull() }
         assertThat(params.modelNameOrNull()).isNull()
     }
+
+    @Test
+    fun modelUnionStringNotNull() {
+        val params = ResponseCreateParams.builder().model("my-model").input("Hello, world!").build()
+
+        assertThat(params.modelNameOrNull()).isEqualTo("my-model")
+    }
+
+    @Test
+    fun modelUnionChatNotNull() {
+        val params =
+            ResponseCreateParams.builder().model(ChatModel.GPT_4O).input("Hello, world!").build()
+
+        assertThat(params.modelNameOrNull()).isEqualTo("gpt-4o")
+    }
+
+    @Test
+    fun modelUnionOnlyNotNull() {
+        val params =
+            ResponseCreateParams.builder()
+                .model(ResponsesModel.ResponsesOnlyModel.O1_PRO)
+                .input("Hello, world!")
+                .build()
+
+        assertThat(params.modelNameOrNull()).isEqualTo("o1-pro")
+    }
 }