feat(java): expose the instances used to create Futures (#5)

This adds a `ChatFutures.getChatHistory()` helper method for Java
developers to be able to obtain the chat history. This new method should
be equivalent to Kotlin's `Chat.history`.

.changes/cemetery-caption-bird-authority.json

@@ -0,0 +1 @@
+{"type":"MINOR","changes":["Expose the instances used to create Future APIs in java"]}

generativeai/src/main/java/com/google/ai/client/generativeai/java/ChatFutures.kt

@@ -46,12 +46,17 @@ abstract class ChatFutures internal constructor() {
    */
   abstract fun sendMessageStream(prompt: Content): Publisher<GenerateContentResponse>
 
-  private class FuturesImpl(val chat: Chat) : ChatFutures() {
+  /** Returns the [Chat] instance that was used to create this instance */
+  abstract fun getChat(): Chat
+
+  private class FuturesImpl(private val chat: Chat) : ChatFutures() {
     override fun sendMessage(prompt: Content): ListenableFuture<GenerateContentResponse> =
       SuspendToFutureAdapter.launchFuture { chat.sendMessage(prompt) }
 
     override fun sendMessageStream(prompt: Content): Publisher<GenerateContentResponse> =
       chat.sendMessageStream(prompt).asPublisher()
+
+    override fun getChat(): Chat = chat
   }
 
   companion object {

generativeai/src/main/java/com/google/ai/client/generativeai/java/GenerativeModelFutures.kt

@@ -64,7 +64,10 @@ abstract class GenerativeModelFutures internal constructor() {
    */
   abstract fun startChat(history: List<Content>): ChatFutures
 
-  private class FuturesImpl(val model: GenerativeModel) : GenerativeModelFutures() {
+  /** Returns the [GenerativeModel] instance that was used to create this object */
+  abstract fun getGenerativeModel(): GenerativeModel
+
+  private class FuturesImpl(private val model: GenerativeModel) : GenerativeModelFutures() {
     override fun generateContent(
       vararg prompt: Content
     ): ListenableFuture<GenerateContentResponse> =
@@ -79,6 +82,8 @@ abstract class GenerativeModelFutures internal constructor() {
     override fun startChat(): ChatFutures = startChat(emptyList())
 
     override fun startChat(history: List<Content>): ChatFutures = from(model.startChat(history))
+
+    override fun getGenerativeModel(): GenerativeModel = model
   }
 
   companion object {