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/0001-privileged-mode-gates-writes.md

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+# Privileged mode gates system-file writes
+
+The application starts read-only and permits changes to `/etc/hosts` only after
+sudo access has been validated and a timestamped backup has been created. All
+privileged writes go through `HostsManager` during that mode. This makes the
+authorization boundary visible to the user and guarantees a pre-edit safety
+snapshot; the temporary backup is not a user-facing recovery system.

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

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+# 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.