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_telemetry.py

@@ -0,0 +1,55 @@
+# 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 Telemetry & Span Lifecycle Tracing."""
+
+import pytest
+from pathlib import Path
+from artemis.telemetry.models import SpanType
+from artemis.telemetry.tracer import TelemetryTracer
+
+
+@pytest.mark.asyncio
+async def test_telemetry_span_lifecycle(tmp_path: Path):
+    """Verify TelemetryTracer properly records and persists nested spans."""
+    tracer = TelemetryTracer(trace_id="test-session-001", traces_dir=tmp_path)
+
+    async with tracer.start_span("parent_agent_step", span_type=SpanType.AGENT) as parent_span:
+        assert parent_span.status == "running"
+        assert parent_span.name == "parent_agent_step"
+
+        async with tracer.start_span(
+            "child_device_action", span_type=SpanType.DEVICE_ACTION, payload={"action": "click"}
+        ) as child_span:
+            assert child_span.parent_id == parent_span.span_id
+            assert child_span.payload["action"] == "click"
+
+    # Spans should be persisted in JSONL
+    assert len(tracer.spans) == 2
+    child_recorded = tracer.spans[0]
+    parent_recorded = tracer.spans[1]
+
+    assert child_recorded.name == "child_device_action"
+    assert child_recorded.status == "success"
+    assert child_recorded.duration_ms is not None
+
+    assert parent_recorded.name == "parent_agent_step"
+    assert parent_recorded.status == "success"
+
+    # File existence
+    log_file = tmp_path / "telemetry_spans.jsonl"
+    assert log_file.exists()
+    content = log_file.read_text(encoding="utf-8")
+    assert "child_device_action" in content
+    assert "parent_agent_step" in content