fix: add missing Apache 2.0 license header to test_readonly_context_state.py

The header-check CI was failing because this new test file was missing
the required copyright/license header.

Author: timof1308

tests/unittests/agents/test_readonly_context_state.py

@@ -1,3 +1,17 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
 from collections import ChainMap
 import unittest
 from unittest.mock import MagicMock