[AI] Improve error messaging for LiveServerMessageSerializer (#7180)

The error message should include the actual content of the message that
couldn't be de-serialized instead of class around java class names,
which are not useful before de-serialization succeds.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: rlazo

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

@@ -183,7 +183,7 @@ internal object LiveServerMessageSerializer :
         LiveServerToolCallCancellation.InternalWrapper.serializer()
       else ->
         throw SerializationException(
-          "The given subclass of LiveServerMessage (${javaClass.simpleName}) is not supported in the serialization yet."
+          "Unknown LiveServerMessage response type. Keys found: ${jsonObject.keys}"
         )
     }
   }