fix: cz bump subdir path​

[Yusin0903]

Description

This pull request fixes the construction of the changelog file name in changelog.py to ensure type safety and correct path handling. It also adds proper error handling for missing or invalid changelog file names.

Checklist

• Add test cases to all the changes you introduce
• Run poetry all locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

The changelog command should now correctly resolve the changelog file path, regardless of whether the config path is set, and should raise a clear error if the file name is missing or invalid.

Steps to Test This Pull Request

1. Run poetry all and ensure all checks pass.
2. Run the changelog command with and without the --file-name argument.
3. Verify that the changelog file is created at the expected location.
4. Check that an error is raised if the changelog file name is missing or invalid.

Additional context