test(spanner): disable Mockito annotation copying for Credentials, ServiceAccountCredentials, and RequestMetadataCallback

[nnicolee]

Problem

In JSpecify 1.0.0, the @NullMarked annotation targets ElementType.MODULE. Because ElementType.MODULE was introduced in Java 9, reflecting on @NullMarked classes under Java 8 (JDK 1.8) throws EnumConstantNotPresentExceptionProxy wrapped in an ArrayStoreException.

In MutableCredentialsTest and SpannerPoolTest, mock declarations for Credentials, ServiceAccountCredentials, and RequestMetadataCallback were created using the default mock(Class.class) without setting .withoutAnnotations(). Mockito attempts to copy annotations on the mock subclasses, triggering the reflection crash on the JSpecify @NullMarked annotation during Java 8 CI pipelines.

Solution

Replaced default mock(...) calls with mock(..., withSettings().withoutAnnotations()) for:

• Credentials in SpannerPoolTest
• ServiceAccountCredentials and RequestMetadataCallback in MutableCredentialsTest

This disables annotation copying on these mocked instances and bypasses the Java 8 reflection limitations.

[gemini-code-assist]

Code Review

This pull request updates mock creation in MutableCredentialsTest and SpannerPoolTest to use Mockito's withSettings().withoutAnnotations() when mocking credentials classes. There are no review comments, and I have no feedback to provide.