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.

CONTEXT.md

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+# Hosts Management
+
+This context describes the language used by the terminal application that
+manages the system hosts file. These terms distinguish operating-system host
+mappings from application modes and DNS-derived mappings.
+
+## Language
+
+**Hosts File**:
+The system file containing local mappings from IP addresses to hostnames.
+_Avoid_: Host database, configuration file
+
+**Host Entry**:
+One IP-address mapping with one or more hostnames and an optional comment.
+_Avoid_: Record, row
+
+**Canonical Hostname**:
+The first hostname in a Host Entry and the primary name displayed for it.
+_Avoid_: Primary domain, main host
+
+**Alias**:
+Any hostname after the Canonical Hostname in the same Host Entry.
+_Avoid_: Secondary domain, alternate entry
+
+**Active Entry**:
+A Host Entry whose mapping participates in host resolution because its line is
+not commented out.
+_Avoid_: Enabled host
+
+**Inactive Entry**:
+A Host Entry retained in the Hosts File as a commented-out mapping.
+_Avoid_: Disabled host, deleted entry
+
+**Default Entry**:
+A protected Host Entry representing a baseline localhost or broadcasthost
+mapping expected by the operating system.
+_Avoid_: Built-in row, standard record
+
+**DNS Entry**:
+A Host Entry whose IP address is derived from a configured DNS name and then
+stored as a normal hosts mapping.
+_Avoid_: CNAME, dynamic host
+
+**Resolved IP**:
+The IP address most recently obtained for a DNS Entry's configured DNS name.
+_Avoid_: Lookup result, dynamic IP
+
+**Read-only Mode**:
+The normal application state in which the Hosts File can be inspected but not
+changed.
+_Avoid_: View mode, safe mode
+
+**Privileged Mode**:
+The application state in which system authorization has been validated and
+Host Entries may be changed.
+_Avoid_: Edit mode, sudo mode
+
+**Pre-edit Backup**:
+A timestamped copy of the Hosts File created before Privileged Mode begins.
+_Avoid_: Session backup, safety snapshot
+
+**Entry Editor**:
+The form for changing one Host Entry while the application is in Privileged
+Mode.
+_Avoid_: Edit mode, detail mode