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_prompt_builder.py

@@ -0,0 +1,56 @@
+# 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 OperatorPromptBuilder and LoopDetector."""
+
+from artemis.agents.operator.loop_detector import LoopDetector
+from artemis.agents.operator.prompt_builder import OperatorPromptBuilder
+
+
+def test_operator_prompt_builder():
+    """Verify prompt rendering formats variables correctly."""
+    system_msg = OperatorPromptBuilder.build_system_message()
+    assert "ARTEMIS Operator" in system_msg
+    assert "click" in system_msg
+
+    human_msg = OperatorPromptBuilder.build_human_message(
+        goal="Open Settings",
+        sub_goal="Tap Network & internet",
+        current_turn=2,
+        history="1. Tapped Settings icon",
+    )
+    assert "Open Settings" in human_msg
+    assert "Tap Network & internet" in human_msg
+    assert "Current Turn: 2" in human_msg
+
+
+def test_loop_detector():
+    """Verify loop detector flags repeated action sequences."""
+    detector = LoopDetector(repetition_threshold=3)
+    assert not detector.is_loop_detected()
+
+    # Record distinct actions
+    detector.record_action("click", {"target": [100, 200]})
+    detector.record_action("swipe", {"action": "up"})
+    assert not detector.is_loop_detected()
+
+    # Record 3 identical actions
+    detector.record_action("click", {"target": [500, 500]})
+    detector.record_action("click", {"target": [500, 500]})
+    detector.record_action("click", {"target": [500, 500]})
+    assert detector.is_loop_detected()
+
+    # Reset
+    detector.reset()
+    assert not detector.is_loop_detected()