Add PR number to changelog entry

Author: rlazo

firebase-ai/CHANGELOG.md

@@ -1,7 +1,7 @@
 # Unreleased
 
 - [feature] Added support for configuring thinking levels with Gemini 3 series
-  mdels and onwards.
+  mdels and onwards. (#7599)
 - [changed] Added `LiveAudioConversationConfig` to control different aspects of the conversation
   while using the `startAudioConversation` function.
 - [fixed] Fixed an issue causing streaming chat interactions to drop thought signatures. (#7562)