Python 3.12 support and exatest cleanup

[tkilias]

Summary

• Upgrade the project to Python 3.12 across packaging, CI, and test tooling.
• Update the UDF debug path for Python 3.12 compatibility.
• Remove the obsolete exatest server stack and related test helpers, including FTP, SMTP, HTTP, MessageBox, and the standalone threading helper.
• Keep the remaining exatest surface focused on the UDF-related pieces still used by the test framework.

Validation from PR

• limactl shell default with Poetry on Python 3.12
• nox -s lint
• Integration tests executed against the ITDE docker-db in Lima
• FTP, SMTP, UDF debug, and docker-db environment checks passed before the server cleanup