[8.2] Resolve javac warnings in perc-checkboxtree module #2044

Description
The perc-checkboxtree module currently completes its build successfully, but still reports Java compiler (javac) and/or Javadoc source warnings that should be cleaned up to improve code quality and maintain a warning-free build.

Cleaning these issues will improve API documentation consistency and help maintain overall code quality.

Module

perc-checkboxtree

Build Summary

• Module: perc-checkboxtree
• Build Status: SUCCESS
• Reactor Index: 48
• Total Diagnostics: 10
• Javac Warnings: 10 (unique: 10)
• Javac Errors: 0
• Javadoc Warnings: 0
• Javadoc Errors: 0
• Analysis Warnings: 0
• Analysis Errors: 0

Expected Result

• The module should build without any javac or javadoc warnings or errors.
• All source files should compile cleanly under JDK 21 with the project's -Xlint:all settings.
• Public classes, interfaces, methods, constructors, fields, and parameters should contain complete and valid Javadoc.

Javadoc generation should complete cleanly without warnings.

Actual Result

The module builds with status SUCCESS, but 10 diagnostic message(s) are reported during compilation.

Acceptance Criteria

• Resolve all javac and javadoc warnings in the perc-checkboxtree module.
• Ensure all public APIs have complete and valid Javadoc.
• Verify the module builds successfully with zero javac errors, zero javadoc errors, and zero javadoc warnings.
• Confirm no regressions are introduced during cleanup.

Documents to refer and follow:
java-api-writing-specs.md
javadoc-checklist.md
javadoc-spec-summary.md
javadoc-tool-architecture.md
javadoc-tool-comments.md