Add interactive step testing to Tool Builder

- New TestStepDialog for testing individual steps from the GUI
- Test button in Tool Builder to test selected step
- Auto-detects variables from step templates
- Multiple assertion types: not_empty, contains, valid_json,
  matches_regex, min/max_length, equals, valid_python
- Background execution with timing metrics
- Provider override option for testing with mock provider
- Output variable display and assertion pass/fail results

Completes M5: Testing & Polish milestone (100%)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/cmdforge/gui/pages/tool_builder_page.py

@@ -233,6 +233,12 @@ class ToolBuilderPage(QWidget):
         self.btn_edit_step.clicked.connect(self._edit_step)
         steps_btns.addWidget(self.btn_edit_step)
 
+        self.btn_test_step = QPushButton("Test")
+        self.btn_test_step.setObjectName("secondary")
+        self.btn_test_step.setToolTip("Test the selected step with custom input")
+        self.btn_test_step.clicked.connect(self._test_step)
+        steps_btns.addWidget(self.btn_test_step)
+
         self.btn_del_step = QPushButton("Delete")
         self.btn_del_step.setObjectName("danger")
         self.btn_del_step.setToolTip("Delete the selected step")
@@ -691,6 +697,21 @@ class ToolBuilderPage(QWidget):
                 self._add_tool_dependency(new_step.tool)
             self._refresh_steps()
 
+    def _test_step(self):
+        """Test the selected step with custom input."""
+        items = self.steps_list.selectedItems()
+        if not items:
+            QMessageBox.information(self, "Test Step", "Please select a step to test")
+            return
+
+        step = items[0].data(Qt.UserRole)
+        idx = self.steps_list.row(items[0])
+        available_vars = self._get_available_vars(up_to_step=idx)
+
+        from ..dialogs.test_step_dialog import TestStepDialog
+        dialog = TestStepDialog(self, step, available_vars=available_vars)
+        dialog.exec()
+
     def _delete_step(self):
         """Delete selected step."""
         items = self.steps_list.selectedItems()