Add unit tests for HostEntry and HostsFile models, and implement HostsParser tests

- Created comprehensive unit tests for HostEntry class, covering creation, validation, and conversion to/from hosts file lines.
- Developed unit tests for HostsFile class, including entry management, sorting, and retrieval of active/inactive entries.
- Implemented tests for HostsParser class, validating parsing and serialization of hosts files, handling comments, and file operations.
- Ensured coverage for edge cases such as empty files, invalid entries, and file permission checks.

tests/__init__.py

@@ -0,0 +1,6 @@
+"""
+Test suite for the hosts TUI application.
+
+This module contains unit tests, integration tests, and TUI tests
+for validating the functionality of the hosts manager.
+"""