[Ai] Expose APINotConfiguredException in the public API (#7259)

The exception was added in #7233 but was mistakenly marked as internal.

Author: rlazo

firebase-ai/api.txt

@@ -157,6 +157,9 @@ package com.google.firebase.ai.java {
 
 package com.google.firebase.ai.type {
 
+  public final class APINotConfiguredException extends com.google.firebase.ai.type.FirebaseAIException {
+  }
+
   public final class AudioRecordInitializationFailedException extends com.google.firebase.ai.type.FirebaseAIException {
     ctor public AudioRecordInitializationFailedException(String message);
   }

firebase-ai/src/main/kotlin/com/google/firebase/ai/type/Exceptions.kt

@@ -158,7 +158,7 @@ public class UnsupportedUserLocationException internal constructor(cause: Throwa
  * [steps](https://firebase.google.com/docs/ai-logic/faq-and-troubleshooting?api=dev#error-genai-config-not-found)
  * to enable the Gemini Developer API.
  */
-internal class APINotConfiguredException internal constructor(cause: Throwable? = null) :
+public class APINotConfiguredException internal constructor(cause: Throwable? = null) :
   FirebaseAIException("Gemini Developer API not enabled in Firebase console.", cause)
 
 /**