fix(api): adding variable to variable pool recursively while loading draft variables. (#21478)

This PR fix the issue that `ObjectSegment` are not recursively added to the draft variable pool while loading draft variables from database. It also fixes an issue about loading variables with more than two elements in the its selector.

Enhances #19735.
Closes #21477.

api/tests/unit_tests/core/workflow/utils/test_variable_utils.py

@@ -0,0 +1,148 @@
+from typing import Any
+
+from core.variables.segments import ObjectSegment, StringSegment
+from core.workflow.entities.variable_pool import VariablePool
+from core.workflow.utils.variable_utils import append_variables_recursively
+
+
+class TestAppendVariablesRecursively:
+    """Test cases for append_variables_recursively function"""
+
+    def test_append_simple_dict_value(self):
+        """Test appending a simple dictionary value"""
+        pool = VariablePool()
+        node_id = "test_node"
+        variable_key_list = ["output"]
+        variable_value = {"name": "John", "age": 30}
+
+        append_variables_recursively(pool, node_id, variable_key_list, variable_value)
+
+        # Check that the main variable is added
+        main_var = pool.get([node_id] + variable_key_list)
+        assert main_var is not None
+        assert main_var.value == variable_value
+
+        # Check that nested variables are added recursively
+        name_var = pool.get([node_id] + variable_key_list + ["name"])
+        assert name_var is not None
+        assert name_var.value == "John"
+
+        age_var = pool.get([node_id] + variable_key_list + ["age"])
+        assert age_var is not None
+        assert age_var.value == 30
+
+    def test_append_object_segment_value(self):
+        """Test appending an ObjectSegment value"""
+        pool = VariablePool()
+        node_id = "test_node"
+        variable_key_list = ["result"]
+
+        # Create an ObjectSegment
+        obj_data = {"status": "success", "code": 200}
+        variable_value = ObjectSegment(value=obj_data)
+
+        append_variables_recursively(pool, node_id, variable_key_list, variable_value)
+
+        # Check that the main variable is added
+        main_var = pool.get([node_id] + variable_key_list)
+        assert main_var is not None
+        assert isinstance(main_var, ObjectSegment)
+        assert main_var.value == obj_data
+
+        # Check that nested variables are added recursively
+        status_var = pool.get([node_id] + variable_key_list + ["status"])
+        assert status_var is not None
+        assert status_var.value == "success"
+
+        code_var = pool.get([node_id] + variable_key_list + ["code"])
+        assert code_var is not None
+        assert code_var.value == 200
+
+    def test_append_nested_dict_value(self):
+        """Test appending a nested dictionary value"""
+        pool = VariablePool()
+        node_id = "test_node"
+        variable_key_list = ["data"]
+
+        variable_value = {
+            "user": {
+                "profile": {"name": "Alice", "email": "alice@example.com"},
+                "settings": {"theme": "dark", "notifications": True},
+            },
+            "metadata": {"version": "1.0", "timestamp": 1234567890},
+        }
+
+        append_variables_recursively(pool, node_id, variable_key_list, variable_value)
+
+        # Check deeply nested variables
+        name_var = pool.get([node_id] + variable_key_list + ["user", "profile", "name"])
+        assert name_var is not None
+        assert name_var.value == "Alice"
+
+        email_var = pool.get([node_id] + variable_key_list + ["user", "profile", "email"])
+        assert email_var is not None
+        assert email_var.value == "alice@example.com"
+
+        theme_var = pool.get([node_id] + variable_key_list + ["user", "settings", "theme"])
+        assert theme_var is not None
+        assert theme_var.value == "dark"
+
+        notifications_var = pool.get([node_id] + variable_key_list + ["user", "settings", "notifications"])
+        assert notifications_var is not None
+        assert notifications_var.value == 1  # Boolean True is converted to integer 1
+
+        version_var = pool.get([node_id] + variable_key_list + ["metadata", "version"])
+        assert version_var is not None
+        assert version_var.value == "1.0"
+
+    def test_append_non_dict_value(self):
+        """Test appending a non-dictionary value (should not recurse)"""
+        pool = VariablePool()
+        node_id = "test_node"
+        variable_key_list = ["simple"]
+        variable_value = "simple_string"
+
+        append_variables_recursively(pool, node_id, variable_key_list, variable_value)
+
+        # Check that only the main variable is added
+        main_var = pool.get([node_id] + variable_key_list)
+        assert main_var is not None
+        assert main_var.value == variable_value
+
+        # Ensure no additional variables are created
+        assert len(pool.variable_dictionary[node_id]) == 1
+
+    def test_append_segment_non_object_value(self):
+        """Test appending a Segment that is not ObjectSegment (should not recurse)"""
+        pool = VariablePool()
+        node_id = "test_node"
+        variable_key_list = ["text"]
+        variable_value = StringSegment(value="Hello World")
+
+        append_variables_recursively(pool, node_id, variable_key_list, variable_value)
+
+        # Check that only the main variable is added
+        main_var = pool.get([node_id] + variable_key_list)
+        assert main_var is not None
+        assert isinstance(main_var, StringSegment)
+        assert main_var.value == "Hello World"
+
+        # Ensure no additional variables are created
+        assert len(pool.variable_dictionary[node_id]) == 1
+
+    def test_append_empty_dict_value(self):
+        """Test appending an empty dictionary value"""
+        pool = VariablePool()
+        node_id = "test_node"
+        variable_key_list = ["empty"]
+        variable_value: dict[str, Any] = {}
+
+        append_variables_recursively(pool, node_id, variable_key_list, variable_value)
+
+        # Check that the main variable is added
+        main_var = pool.get([node_id] + variable_key_list)
+        assert main_var is not None
+        assert main_var.value == {}
+
+        # Ensure only the main variable is created (no recursion for empty dict)
+        assert len(pool.variable_dictionary[node_id]) == 1