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_gemini_role.py

@@ -0,0 +1,67 @@
+# 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 google import genai
+from google.genai import types
+from google.genai.errors import APIError
+import pytest
+
+
+@pytest.fixture
+def client():
+    """Initializes the Gemini client. Requires GEMINI_API_KEY in environment."""
+    try:
+        return genai.Client()
+    except Exception as e:
+        pytest.skip(f"Could not initialize genai.Client: {e}")
+
+
+def test_gemini_role_tool_fails(client):
+    """Confirm that using role='tool' results in a 400 APIError from Gemini."""
+    contents = [
+        types.Content(
+            role="tool",
+            parts=[types.Part.from_text(text="This is a test payload for tool execution.")],
+        )
+    ]
+
+    with pytest.raises(APIError) as exc_info:
+        client.models.generate_content(
+            model="gemini-3.6-flash",
+            contents=contents,
+        )
+
+    # Assert it failed specifically due to the invalid role string
+    error_msg = str(exc_info.value)
+    assert "Invalid role string" in error_msg or "400" in error_msg
+
+
+def test_gemini_role_user_succeeds(client):
+    """Confirm that using role='user' (which is the correct fix) succeeds."""
+    contents = [
+        types.Content(
+            role="user",
+            parts=[
+                types.Part.from_text(text="Acknowledge this message with 'ok' and nothing else.")
+            ],
+        )
+    ]
+
+    # This should not raise any exceptions
+    response = client.models.generate_content(
+        model="gemini-3.6-flash",
+        contents=contents,
+    )
+
+    assert response.text is not None