test(monitoring): add tests for Flask backend version endpoint

- Add pytest to requirements.txt
- Add tests for /version endpoint (returns JSON, contains version and build_ts)
- Add tests for read_version_file function (success, whitespace handling, file not found)
- Add tests for /health endpoint (healthy and unhealthy states)

Author: claude

monitoring_flask_backend/requirements.txt

@@ -2,4 +2,5 @@ Flask==3.0.0
 prometheus-api-client==0.5.4
 python-dateutil==2.8.2
 gunicorn==21.2.0
-requests==2.31.0 
+requests==2.31.0
+pytest==8.3.4 

monitoring_flask_backend/test_app.py

@@ -0,0 +1,88 @@
+"""Tests for the Flask monitoring backend."""
+import pytest
+import json
+from unittest.mock import patch, mock_open
+
+from app import app, read_version_file
+
+
+@pytest.fixture
+def client():
+    """Create test client."""
+    app.config['TESTING'] = True
+    with app.test_client() as client:
+        yield client
+
+
+class TestVersionEndpoint:
+    """Tests for the /version endpoint."""
+
+    def test_version_endpoint_returns_json(self, client):
+        """Test that /version returns valid JSON."""
+        response = client.get('/version')
+        assert response.status_code == 200
+        assert response.content_type == 'application/json'
+
+    def test_version_endpoint_contains_version_field(self, client):
+        """Test that /version response contains version field."""
+        response = client.get('/version')
+        data = json.loads(response.data)
+        assert 'version' in data
+
+    def test_version_endpoint_contains_build_ts_field(self, client):
+        """Test that /version response contains build_ts field."""
+        response = client.get('/version')
+        data = json.loads(response.data)
+        assert 'build_ts' in data
+
+
+class TestReadVersionFile:
+    """Tests for the read_version_file function."""
+
+    def test_read_version_file_success(self):
+        """Test reading version file successfully."""
+        mock_content = "1.2.3"
+        with patch("builtins.open", mock_open(read_data=mock_content)):
+            result = read_version_file("/VERSION")
+            assert result == "1.2.3"
+
+    def test_read_version_file_strips_whitespace(self):
+        """Test that version file content is stripped."""
+        mock_content = "  1.2.3\n  "
+        with patch("builtins.open", mock_open(read_data=mock_content)):
+            result = read_version_file("/VERSION")
+            assert result == "1.2.3"
+
+    def test_read_version_file_not_found_returns_default(self):
+        """Test that missing file returns default value."""
+        with patch("builtins.open", side_effect=FileNotFoundError()):
+            result = read_version_file("/VERSION")
+            assert result == "unknown"
+
+    def test_read_version_file_custom_default(self):
+        """Test custom default value when file not found."""
+        with patch("builtins.open", side_effect=FileNotFoundError()):
+            result = read_version_file("/VERSION", default="0.0.0")
+            assert result == "0.0.0"
+
+
+class TestHealthEndpoint:
+    """Tests for the /health endpoint."""
+
+    @patch('app.get_prometheus_client')
+    def test_health_endpoint_healthy(self, mock_prom, client):
+        """Test /health returns healthy when Prometheus is reachable."""
+        mock_prom.return_value.get_current_metric_value.return_value = [{'value': 1}]
+        response = client.get('/health')
+        assert response.status_code == 200
+        data = json.loads(response.data)
+        assert data['status'] == 'healthy'
+
+    @patch('app.get_prometheus_client')
+    def test_health_endpoint_unhealthy(self, mock_prom, client):
+        """Test /health returns unhealthy when Prometheus is unreachable."""
+        mock_prom.return_value.get_current_metric_value.side_effect = Exception("Connection failed")
+        response = client.get('/health')
+        assert response.status_code == 500
+        data = json.loads(response.data)
+        assert data['status'] == 'unhealthy'