Refactor test cases for improved readability and consistency

- Updated test_dns.py to enhance mock function definitions and improve spacing for better readability.
- Modified test_filters.py to streamline assertions and ensure consistent formatting across test cases.
- Cleaned up test_import_export.py by organizing imports and ensuring consistent formatting in CSV and JSON tests.
- Improved test_main.py by refining mock setups and ensuring consistent error handling in assertions.

docs/adr/0002-normalize-hosts-serialization.md

@@ -0,0 +1,7 @@
+# Normalize hosts-file serialization
+
+The parser models Host Entries plus header and footer comments rather than the
+file's exact byte layout. Serialization deliberately aligns fields, groups
+comments, omits original blank-line placement, and ensures the management header
+is present. This trades lossless round trips for a stable, readable output; code
+and documentation must not promise preservation of original formatting.