Refactor and correct the codemetropolis-toolchain-commons

[premissa]

CodeMetropolis was chosen as a project in the Software Testing course. The students tested the application and found some issues that should be checked and corrected.

SonarLint provided the following results:

The current task is the following:

• This task is a refactoring. Before starting working on this task, review the refactoring parts of our curriculum. See: https://okt.inf.szte.hu/rf2/tananyag/08.html
• Focus on only major and critical issues!
• Review all issues mentioned before modifying them.
• Define constant instead of duplicating this literal.
• Add a default case to switches.
• Refactor methods to reduce its Cognitive Complexity.
• Disable access to external entities in XML parsing.

Acceptance criteria:

The issues enumerated in the previous paragraph have been reviewed, and the necessary corrections have been made.