Improve serialVersionUID check in tests

[johnycho]

Fixes gh-17729

Description

The allSerializableClassesShouldHaveSerialVersionOrSuppressWarnings test logic had two issues that caused it to pass incorrectly (false positives):

1. Logic Error

It permitted classes to pass if the annotation was null (suppressWarnings == null).
Since @SuppressWarnings has RetentionPolicy.SOURCE, it is not visible at runtime via reflection, causing getAnnotation to always return null. Consequently, the test was skipping validation for all classes.

2. Typo

It checked for "Serial" (case-sensitive) instead of "serial".

Fix

This PR updates the validation logic to:

• Correct the value to "serial".
• Enforce a strict check by ensuring the class is ignored only if the annotation is explicitly present (non-null) and contains the correct value.

Verification

To demonstrate the fix, I temporarily removed serialVersionUID from a class (e.g., SimpleGrantedAuthority) and ran the test.

State	Screenshot	Result
Before Fix		PASSED (False Positive) ❌ The test incorrectly passed even though serialVersionUID was missing, because the verification logic was broken.
After Fix		FAILED (Correct Behavior) ✅ The test now correctly detects the missing serialVersionUID and fails, enforcing the strict check as intended.

Note

This change fixes the test logic to be correct. However, due to the RetentionPolicy.SOURCE limitation of @SuppressWarnings, this strict check might cause the test to fail for classes that rely on @SuppressWarnings("serial") without a serialVersionUID. This reveals that the previous test was not actually verifying those classes.