backup: #904 v17 - add buf.Len() == 0 assertions to Redis rejection tests

Claude v16 observation (non-blocking but worth folding in): the two
v16 rejection tests TestEncodeSnapshotRedisRejectsNonZeroDB and
TestEncodeSnapshotRedisRejectsMultipleDBs pass a *bytes.Buffer to
EncodeSnapshot but never assert it stays empty on rejection.

All five prior rejection tests consistently include the assertion:
- TestEncodeSnapshotRejectsMissingInputRoot
- TestEncodeSnapshotRejectsLowManifestFloor
- TestEncodeSnapshotRejectsDynamoDBJSONLLayout
- TestEncodeSnapshotRejectsZeroAdapterSet
- TestEncodeSnapshotMarksAdapterDataErrors

Adding the same `if buf.Len() != 0 { t.Errorf(...) }` block to both
new tests keeps the convention consistent and pins the
no-bytes-on-rejection invariant explicitly.

No behavior change; tests + lint green.

Author: bootjp

internal/backup/encode_snapshot_test.go

@@ -498,6 +498,9 @@ func TestEncodeSnapshotRedisRejectsNonZeroDB(t *testing.T) {
 	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())
+	}
 }
 
 // TestEncodeSnapshotRedisRejectsMultipleDBs pins the multi-DB case:
@@ -524,6 +527,9 @@ func TestEncodeSnapshotRedisRejectsMultipleDBs(t *testing.T) {
 	if !errors.Is(err, ErrRedisEncodeMultiDBUnsupported) {
 		t.Errorf("err = %v, want errors.Is ErrRedisEncodeMultiDBUnsupported", err)
 	}
+	if buf.Len() != 0 {
+		t.Errorf("buf.Len = %d, want 0 (no bytes should be written on multi-DB rejection)", buf.Len())
+	}
 }
 
 // TestEnumerateRedisDBsRejectsNonDirDBEntry pins codex P2 v14 #904