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_planner.py

@@ -0,0 +1,46 @@
+# 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 asyncio
+from pathlib import Path
+from artemis.agents.planner.planner import run_async_planner_validation
+
+
+class DummyDataEngine:
+    def get_agent_friendly_steps(self):
+        return []
+
+
+class DummyCtx:
+    data_engine = DummyDataEngine()
+    project_dir = str(Path(__file__).resolve().parent.parent.parent)
+
+
+async def test():
+    content_before = "- [/] Click the button\n- [ ] Complete task"
+    content_after = (
+        "- [/] Click the button\n- [ ] Complete task\n- [ ] New subgoal appended at bottom"
+    )
+    res = await run_async_planner_validation(
+        ctx=DummyCtx(),
+        initial_goal="Do the task",
+        content_before=content_before,
+        content_after=content_after,
+        operator_raw_thinking="I need to add a new subgoal",
+        operator_native_thinking="Thinking...",
+    )
+    print(res)
+
+
+asyncio.run(test())