fix(api): cap the client traffic list a master pushes by entry count

pushClientTraffics decoded the pushed list whole, and each "{}" became a
full traffic row, so a body under the size cap could cost the panel
close to a gigabyte. The list is now decoded entry by entry and refused
past 262144 entries. The capped decoder the node runtime already used
moves to json_util so both share it.

internal/util/json_util/list.go

@@ -0,0 +1,38 @@
+package json_util
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+)
+
+// ErrListTooLong is returned by DecodeCappedList for a list past its entry limit.
+var ErrListTooLong = errors.New("list exceeds entry limit")
+
+// DecodeCappedList decodes a JSON array entry by entry and stops at the first entry past
+// maxEntries: each "{}" becomes a whole T, so a byte cap alone does not bound the result.
+func DecodeCappedList[T any](raw []byte, maxEntries int) ([]T, error) {
+	raw = bytes.TrimSpace(raw)
+	if len(raw) == 0 || bytes.Equal(raw, []byte("null")) {
+		return nil, nil
+	}
+	dec := json.NewDecoder(bytes.NewReader(raw))
+	if tok, err := dec.Token(); err != nil {
+		return nil, err
+	} else if delim, ok := tok.(json.Delim); !ok || delim != '[' {
+		return nil, fmt.Errorf("expected a JSON array, got %v", tok)
+	}
+	var out []T
+	for dec.More() {
+		if len(out) >= maxEntries {
+			return nil, fmt.Errorf("%w (%d)", ErrListTooLong, maxEntries)
+		}
+		var item T
+		if err := dec.Decode(&item); err != nil {
+			return nil, err
+		}
+		out = append(out, item)
+	}
+	return out, nil
+}