test: add comprehensive unit, integration, and e2e test suites

Introduce complete test coverage for the ARTEMIS platform across multiple layers:
Unit tests: Add tests for agent nodes (Planner, Operator, Validator, Diagnoser, FlashRunner, Explorer), tools, utils (OCR/XML fusion, UI hierarchy), and data engine.
Integration tests: Add live tool verification, reactive runner, IME, and standalone workflow tests.
E2E tests: Add end-to-end scenario tests for media playback and failure recovery flows.
Fixtures & mocks: Add mock inputs (audio, video, screenshot) and test utilities.

tests/unit/test_update.py

@@ -0,0 +1,36 @@
+# 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.
+
+import os
+from artemis.utils.notes import update_note_content
+
+base_dir = "/tmp/artemis_test"
+os.makedirs(f"{base_dir}/notes", exist_ok=True)
+task_plan = """- [x] Initial task
+- [/] Active task
+- [ ] Future task"""
+with open(f"{base_dir}/notes/task_plan.md", "w") as f:
+    f.write(task_plan)
+
+try:
+    res = update_note_content(
+        base_dir,
+        "task_plan",
+        "- [/] Active task",
+        "- [/] Active task\n    - [ ] New subgoal",
+    )
+    print("Update successful!")
+    print(open(f"{base_dir}/notes/task_plan.md").read())
+except Exception as e:
+    print("Error:", e)