[Vertex AI] Read golden files from vertexai subdirectory (#14582)

Author: andrewheard

FirebaseVertexAI.podspec

@@ -61,7 +61,7 @@ Firebase SDK.
       unit_tests_dir + '**/*.swift',
     ]
     unit_tests.resources = [
-      unit_tests_dir + 'vertexai-sdk-test-data/mock-responses/**/*.{txt,json}',
+      unit_tests_dir + 'vertexai-sdk-test-data/mock-responses/vertexai',
       unit_tests_dir + 'Resources/**/*',
     ]
   end

FirebaseVertexAI/Tests/Unit/ChatTests.swift

@@ -36,7 +36,8 @@ final class ChatTests: XCTestCase {
     let bundle = BundleTestUtil.bundle()
     let fileURL = try XCTUnwrap(bundle.url(
       forResource: "streaming-success-basic-reply-parts",
-      withExtension: "txt"
+      withExtension: "txt",
+      subdirectory: "vertexai"
     ))
 
     // Skip tests using MockURLProtocol on watchOS; unsupported in watchOS 2 and later, see