test(auth): verify GoogleCredentials.fromStream throws IOException on invalid JSON (#13473)

Other client libraries (such as Python, Go, and Rust) strictly validate
JSON syntax and reject malformed payload structures immediately. This
test ensures Java maintains parity by asserting that an IOException is
explicitly thrown when ADC JSON parsing fails, preventing silent
fallbacks.

This fills an untested gap in the Java ADC resolution suite.

Author: westarle

google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/GoogleCredentialsTest.java

@@ -53,6 +53,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.net.URI;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -122,9 +123,20 @@ void fromStream_unknownType_throws() throws IOException {
     }
   }
 
+  @Test
+  void fromStream_invalidJson_throws() throws IOException {
+    // This test ensures Java successfully throws an IOException when ADC JSON parsing
+    // fails, preventing silent fallbacks.
+    MockHttpTransportFactory transportFactory = new MockHttpTransportFactory();
+    try (InputStream stream =
+        new ByteArrayInputStream("invalid-json{".getBytes(StandardCharsets.UTF_8))) {
+      assertThrows(IOException.class, () -> GoogleCredentials.fromStream(stream, transportFactory));
+    }
+  }
+
   @Test
   void fromStream_nullTransport_throws() {
-    InputStream stream = new ByteArrayInputStream("foo".getBytes());
+    InputStream stream = new ByteArrayInputStream("foo".getBytes(StandardCharsets.UTF_8));
     assertThrows(NullPointerException.class, () -> GoogleCredentials.fromStream(stream, null));
   }