[Firebase AI] Upgrade Gemini 1.x models in integration tests (#14956)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: andrewheard

FirebaseAI/Tests/TestApp/Tests/Integration/IntegrationTests.swift

@@ -67,7 +67,7 @@ final class IntegrationTests: XCTestCase {
   func testCountTokens_text() async throws {
     let prompt = "Why is the sky blue?"
     model = vertex.generativeModel(
-      modelName: "gemini-1.5-pro",
+      modelName: ModelNames.gemini2Flash,
       generationConfig: generationConfig,
       safetySettings: [
         SafetySetting(harmCategory: .harassment, threshold: .blockLowAndAbove, method: .severity),