[AI] Add missing changelog entry for #7970 (#7977)

The change adds the changelog entry missing from the change in #7970,
which implements `toString` for `InferenceSource`.

Author: rlazo

ai-logic/firebase-ai/CHANGELOG.md

@@ -2,6 +2,7 @@
 
 - [fixed] Fixed an issue causing network timeouts to throw the incorrect exception type, instead of
   `RequestTimeoutException` (#7966)
+- [fixed] Fixed missing `toString()` implemenation for `InferenceSource` (#7970)
 - [fixed] Fixed an issue causing the SDK to throw an exception if an unknown message was received
   from the LiveAPI model, instead of ignoring it (#7975)