feat: Add ChatCompletionsRequest object

This is part of a larger chain of commits for adding chat completion API support to the Apigee model.

PiperOrigin-RevId: 894295669

Author: google-genai-bot

core/src/main/java/com/google/adk/models/ChatCompletionsResponse.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.models.chat;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import java.util.Map;
+
+/** Shared models for Chat Completions Request and Response. */
+@JsonIgnoreProperties(ignoreUnknown = true)
+@JsonInclude(JsonInclude.Include.NON_NULL)
+final class ChatCompletionsCommon {
+
+  private ChatCompletionsCommon() {}
+
+  /**
+   * See
+   * https://developers.openai.com/api/reference/resources/chat#(resource)%20chat.completions%20%3E%20(model)%20chat_completion_message_tool_call%20%3E%20(schema)
+   */
+  @JsonIgnoreProperties(ignoreUnknown = true)
+  @JsonInclude(JsonInclude.Include.NON_NULL)
+  static class ToolCall {
+    /** See class definition for more details. */
+    public Integer index;
+
+    /** See class definition for more details. */
+    public String id;
+
+    /** See class definition for more details. */
+    public String type;
+
+    /** See class definition for more details. */
+    public Function function;
+
+    /** See class definition for more details. */
+    public Custom custom;
+
+    /**
+     * Used to supply additional parameters for specific models, for example:
+     * https://ai.google.dev/gemini-api/docs/openai#thinking
+     */
+    @JsonProperty("extra_content")
+    public Map<String, Object> extraContent;
+  }
+
+  /**
+   * See
+   * https://developers.openai.com/api/reference/resources/chat#(resource)%20chat.completions%20%3E%20(model)%20chat_completion_message_function_tool_call%20%3E%20(schema)
+   */
+  @JsonIgnoreProperties(ignoreUnknown = true)
+  @JsonInclude(JsonInclude.Include.NON_NULL)
+  static class Function {
+    /** See class definition for more details. */
+    public String name;
+
+    /** See class definition for more details. */
+    public String arguments; // JSON string
+  }
+
+  /**
+   * See
+   * https://developers.openai.com/api/reference/resources/chat#(resource)%20chat.completions%20%3E%20(model)%20chat_completion_custom_tool%20%3E%20(schema)
+   */
+  @JsonIgnoreProperties(ignoreUnknown = true)
+  @JsonInclude(JsonInclude.Include.NON_NULL)
+  static class Custom {
+    /** See class definition for more details. */
+    public String input;
+
+    /** See class definition for more details. */
+    public String name;
+  }
+}