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/agents/test_explorer_universal.py

@@ -0,0 +1,127 @@
+# 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.
+
+"""Unit tests for Universal Explorer and Multi-Model Grounding Engine."""
+
+import json
+from unittest.mock import AsyncMock, MagicMock, Mock, patch
+
+import pytest
+from langchain_core.messages import AIMessage
+
+from artemis.agents.explorer.explorer import (
+    UNIVERSAL_EXPLORER_TOOLS,
+    Explorer,
+)
+from artemis.config import settings
+from artemis.context import ArtemisContext
+from artemis.graph.state import State
+
+
+@pytest.fixture
+def mock_context(tmp_path):
+    ctx = Mock(spec=ArtemisContext)
+    ctx.llm_config = Mock()
+    mock_llm_cfg = Mock()
+    mock_llm_cfg.model = "claude-3-7-sonnet"
+    mock_llm_cfg.temperature = 0.1
+    ctx.llm_config.explorer = mock_llm_cfg
+
+    ctx.device = Mock()
+    ctx.device.device_width = 1080
+    ctx.device.device_height = 2400
+
+    ctx.agent_config = Mock()
+    ctx.agent_config.blacklisted_tools = {}
+
+    ctx.data_engine = Mock()
+    ctx.data_engine.base_dir = tmp_path
+    return ctx
+
+
+def test_explorer_universal_engine_detection(mock_context):
+    """Test that Explorer routes to Universal Engine when non-Gemini model is configured."""
+    with patch.object(settings, "GOOGLE_API_KEY", None):
+        agent = Explorer(mock_context)
+        assert agent.use_native_gemini is False
+        assert agent.model_name == "claude-3-7-sonnet"
+
+
+def test_universal_explorer_tool_schemas():
+    """Verify schema integrity of Universal Explorer tool declarations."""
+    assert len(UNIVERSAL_EXPLORER_TOOLS) == 6
+    names = [t["function"]["name"] for t in UNIVERSAL_EXPLORER_TOOLS]
+    assert "ask_perception_tool" in names
+    assert "detect_objects" in names
+    assert "get_ocr_list" in names
+    assert "ask_image_processor" in names
+    assert "inspect_region" in names
+    assert "submit_answer" in names
+
+
+@pytest.mark.asyncio
+async def test_run_universal_explorer_success(mock_context, tmp_path):
+    """Test universal Explorer tool invocation and submit_answer coordinate parsing."""
+    agent = Explorer(mock_context)
+    agent.use_native_gemini = False
+
+    fake_screenshot = tmp_path / "screenshot.jpg"
+    fake_screenshot.write_bytes(b"fake_image_bytes")
+
+    # Mock tool call response for submit_answer
+    msg_response = AIMessage(
+        content="Submitting grounded elements.",
+        tool_calls=[
+            {
+                "name": "submit_answer",
+                "args": {
+                    "candidates": [
+                        {
+                            "label": "D1",
+                            "coords": [500, 600],
+                            "description": "Settings button",
+                        }
+                    ],
+                    "fallback_message": "",
+                },
+                "id": "exp_call_1",
+            }
+        ],
+    )
+
+    mock_llm = MagicMock()
+    mock_bound = MagicMock()
+    mock_bound.ainvoke = AsyncMock(return_value=msg_response)
+    mock_llm.bind_tools.return_value = mock_bound
+
+    with patch(
+        "artemis.agents.explorer.explorer.get_llm",
+        return_value=mock_llm,
+    ):
+        mock_state = Mock(spec=State)
+        res_str = await agent._run_universal(
+            query="find settings icon",
+            context_feedback="",
+            screenshot_path=str(fake_screenshot),
+            state=mock_state,
+            minimal_list="",
+            version="pro",
+            prompt_template="You are an explorer.",
+            max_iterations=3,
+        )
+
+        res = json.loads(res_str)
+        assert len(res["candidates"]) == 1
+        assert res["candidates"][0]["label"] == "D1"
+        assert res["candidates"][0]["coords"] == [500, 600]