add changelog

Author: David Motsonashvili

firebase-ai/CHANGELOG.md

@@ -5,6 +5,7 @@
 * [changed] Introduced the `Voice` class, which accepts a voice name, and deprecated the `Voices` class.
 * [changed] **Breaking Change**: Updated `SpeechConfig` to take in `Voice` class instead of `Voices` class.
     * **Action Required:** Update all references of `SpeechConfig` initialization to use `Voice` class.
+* [fixed] Fix incorrect model name in count token requests to the developer API backend
 
 
 # 16.0.0