Add unit tests for backup_first_start and run them in CI

tests/test_blocklist_scheduler.py

@@ -0,0 +1,41 @@
+import pytest
+
+import blocklist_scheduler as bs
+
+
+def test_backup_first_start_creates_backup_when_missing(tmp_path, monkeypatch):
+    adguard_yaml = tmp_path / "AdGuardHome.yaml"
+    adguard_yaml.write_text("original: config\n")
+    first_backup = tmp_path / "AdGuardHome.yaml.first-start.bak"
+
+    monkeypatch.setattr(bs, "ADGUARD_YAML", adguard_yaml)
+    monkeypatch.setattr(bs, "FIRST_BACKUP", first_backup)
+
+    bs.backup_first_start()
+
+    assert first_backup.read_text() == "original: config\n"
+
+
+def test_backup_first_start_does_not_overwrite_existing_backup(tmp_path, monkeypatch):
+    adguard_yaml = tmp_path / "AdGuardHome.yaml"
+    adguard_yaml.write_text("new: config\n")
+    first_backup = tmp_path / "AdGuardHome.yaml.first-start.bak"
+    first_backup.write_text("pristine: original\n")
+
+    monkeypatch.setattr(bs, "ADGUARD_YAML", adguard_yaml)
+    monkeypatch.setattr(bs, "FIRST_BACKUP", first_backup)
+
+    bs.backup_first_start()
+
+    assert first_backup.read_text() == "pristine: original\n"
+
+
+def test_backup_first_start_raises_if_adguard_yaml_missing(tmp_path, monkeypatch):
+    adguard_yaml = tmp_path / "AdGuardHome.yaml"
+    first_backup = tmp_path / "AdGuardHome.yaml.first-start.bak"
+
+    monkeypatch.setattr(bs, "ADGUARD_YAML", adguard_yaml)
+    monkeypatch.setattr(bs, "FIRST_BACKUP", first_backup)
+
+    with pytest.raises(FileNotFoundError):
+        bs.backup_first_start()