fix(mcp): resolve ruff E501 and formatting issues to pass pre-commit

- Split long string literal in schema_validator.py line 202 (E501, 94 > 88 chars)
- Apply ruff format auto-fixes to big_number.py, handlebars.py, and test_get_chart_data.py

tests/unit_tests/mcp_service/chart/tool/test_get_chart_data.py

@@ -222,9 +222,9 @@ class TestBigNumberChartFallback:
                 "viz_type": viz_type,
             }
             metrics, _ = _extract_metrics_and_groupby(form_data)
-            assert metrics == [
-                {"label": "plural_metric"}
-            ], f"{viz_type} should use plural metrics"
+            assert metrics == [{"label": "plural_metric"}], (
+                f"{viz_type} should use plural metrics"
+            )
 
     def test_pop_kpi_uses_singular_metric(self):
         """Test that pop_kpi (BigNumberPeriodOverPeriod) uses singular metric."""