Add JUnit tests for validating PDB ID regex

[ChunxiaoB]

This PR adds JUnit tests for validating PDB ID regex behavior.

1. PDB_ID_WITH_SUFFIX_REGEX has been updated to match:

• Standard 4-letter PDB IDs
• 4-letter PDB IDs with an optional valid separator and suffix

2. Unit tests have been added to ensure correct matching for both formats.

[josemduarte]

Thank you for adding tests! See one comment about junit version

[josemduarte]

Since this is the first time we add tests for this project, I think it'd be best to have the newest junit version (now version 6). Transforming the test to the latest version should be quite easy, I think even IntelliJ has a tool to migrate. Otherwise with AI.

[ChunxiaoB]

@josemduarte I upgraded JUnit to the latest version and adapted unit tests. Thanks!

[josemduarte]

LGTM thanks!

[ChunxiaoB]

Thanks @josemduarte