Refactor and modernize test suite for compatibility with recent library changes

[gretelliz]

Update and restructure pytest test suite to align with the latest sdx-lib changes introduced in PR #81

The previous test suite had become largely obsolete after the significant refactoring and API changes merged in #81.

This PR completely restructures and updates the tests to:

• Use modern pytest + pytest-mock patterns consistently
• Cover the current implementation of all main modules
• Mock external dependencies properly (requests, subprocess, socket)
• Handle new response formats, error paths, and parsing logic
• Achieve meaningful coverage on previously untested or broken areas

Key additions and fixes:

• Full coverage of VLAN parsing, range expansion/collapse, available ports calculation (with and without used VLANs)
• Tests for input validation (URLs, names, emails, notifications)
• Proper mocking and assertions for HTTP request/response handling and exceptions
• Network utilities (ping, port checking) with correct exception handling and output verification
• Fixed several broken tests caused by outdated mocks, missing imports, incorrect exception expectations, and mismatched output strings.