backup: #904 v18 - add ErrEncodeAdapterData assertion to multi-DB rejection test

Claude v17 noted a pre-existing parity gap:
TestEncodeSnapshotRedisRejectsMultipleDBs asserts
errors.Is(ErrRedisEncodeMultiDBUnsupported) but not
errors.Is(ErrEncodeAdapterData), while its sibling
TestEncodeSnapshotRedisRejectsNonZeroDB asserts both. Both paths
exit runAdapterEncoders through the same errors.Mark wrap so both
sentinels should hold.

Adding the missing assertion for parity. Test-only; no behavior
change. Tests + lint green.

Author: bootjp

internal/backup/encode_snapshot_test.go

@@ -527,6 +527,11 @@ func TestEncodeSnapshotRedisRejectsMultipleDBs(t *testing.T) {
 	if !errors.Is(err, ErrRedisEncodeMultiDBUnsupported) {
 		t.Errorf("err = %v, want errors.Is ErrRedisEncodeMultiDBUnsupported", err)
 	}
+	// Marked as adapter-data so the CLI routes it to exit-2 (mirrors
+	// TestEncodeSnapshotRedisRejectsNonZeroDB; claude v17 parity).
+	if !errors.Is(err, ErrEncodeAdapterData) {
+		t.Errorf("err = %v, want errors.Is ErrEncodeAdapterData (mark from runAdapterEncoders)", err)
+	}
 	if buf.Len() != 0 {
 		t.Errorf("buf.Len = %d, want 0 (no bytes should be written on multi-DB rejection)", buf.Len())
 	}