Add tests for HarmBlockMethod compatibility with GoogleAI and VertexAI

Added unit tests to `GenerativeModelTesting.kt` to verify that:
1. Using `HarmBlockMethod` with `GoogleAI` backend throws `InvalidStateException`.
2. Using `HarmBlockMethod` with `VertexAI` backend does not throw `InvalidStateException`.

This covers an edge case where `HarmBlockMethod` is only supported by VertexAI.

Author: google-labs-jules[bot]

firebase-ai/src/test/java/com/google/firebase/ai/GenerativeModelTesting.kt

@@ -193,9 +193,7 @@ internal class GenerativeModelTesting {
       )
 
     val exception =
-      shouldThrow<InvalidStateException> {
-        generativeModel.generateContent("my test prompt")
-      }
+      shouldThrow<InvalidStateException> { generativeModel.generateContent("my test prompt") }
 
     exception.message shouldContain "HarmBlockMethod is unsupported by the Google Developer API"
   }