Enable checkstyle && fix issues in entity/agent

[TedaLIEz]

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Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Code compiles correctly with all tests are passed.
• [ ] I've read the contributing guide and followed the recommended practices.
• [ ] Wikis or README have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

If this introduces a breaking change for Hydra Lab users and requires a migration process, please describe the impact and migration path for existing applications below.

• [ ] Yes
• [ ] No

Pull Request Description

Enable checkstyle in common module, wave 1

Please check the type of change your PR introduces:

• [ ] Bugfix
• [ ] Feature
• [ ] Build related changes
• [ ] Refactoring (no functional changes, no api changes)
• [x] Code style update (formatting, renaming) or Documentation content changes
• [ ] Other (please describe):

A few words to explain your changes:

Does this close any currently open issues? Issue ID: #

How you tested it

Please make sure the change is tested, you can test it by adding UTs, do local test and share the screenshots, etc.

Feature UI Screenshots or Technical Design Diagrams

If this is a relatively large or complex change, kick off the discussion by drawing the tech design with PlantUML and explaining why you chose the solution you did and what alternatives you considered, etc...

[hydraxman]

Could you modify the doc contributing guide and provide a list there to list all the enabled rules? Or if there are too many, you may briefly summarise the goal or the recommended coding practices of this change in the doc, and attach some official doc link of checkstyle in the contributing doc to provide clarity. @TedaLIEz