feat: Persist Variables for Enhanced Debugging Workflow (#20699)

This pull request introduces a feature aimed at improving the debugging experience during workflow editing. With the addition of variable persistence, the system will automatically retain the output variables from previously executed nodes. These persisted variables can then be reused when debugging subsequent nodes, eliminating the need for repetitive manual input.

By streamlining this aspect of the workflow, the feature minimizes user errors and significantly reduces debugging effort, offering a smoother and more efficient experience.

Key highlights of this change:

- Automatic persistence of output variables for executed nodes.
- Reuse of persisted variables to simplify input steps for nodes requiring them (e.g., `code`, `template`, `variable_assigner`).
- Enhanced debugging experience with reduced friction.

Closes #19735.

api/tests/unit_tests/models/test_workflow.py

@@ -1,10 +1,15 @@
+import dataclasses
 import json
 from unittest import mock
 from uuid import uuid4
 
 from constants import HIDDEN_VALUE
+from core.file.enums import FileTransferMethod, FileType
+from core.file.models import File
 from core.variables import FloatVariable, IntegerVariable, SecretVariable, StringVariable
-from models.workflow import Workflow, WorkflowNodeExecutionModel
+from core.variables.segments import IntegerSegment, Segment
+from factories.variable_factory import build_segment
+from models.workflow import Workflow, WorkflowDraftVariable, WorkflowNodeExecutionModel, is_system_variable_editable
 
 
 def test_environment_variables():
@@ -163,3 +168,147 @@ class TestWorkflowNodeExecution:
         original = {"a": 1, "b": ["2"]}
         node_exec.execution_metadata = json.dumps(original)
         assert node_exec.execution_metadata_dict == original
+
+
+class TestIsSystemVariableEditable:
+    def test_is_system_variable(self):
+        cases = [
+            ("query", True),
+            ("files", True),
+            ("dialogue_count", False),
+            ("conversation_id", False),
+            ("user_id", False),
+            ("app_id", False),
+            ("workflow_id", False),
+            ("workflow_run_id", False),
+        ]
+        for name, editable in cases:
+            assert editable == is_system_variable_editable(name)
+
+        assert is_system_variable_editable("invalid_or_new_system_variable") == False
+
+
+class TestWorkflowDraftVariableGetValue:
+    def test_get_value_by_case(self):
+        @dataclasses.dataclass
+        class TestCase:
+            name: str
+            value: Segment
+
+        tenant_id = "test_tenant_id"
+
+        test_file = File(
+            tenant_id=tenant_id,
+            type=FileType.IMAGE,
+            transfer_method=FileTransferMethod.REMOTE_URL,
+            remote_url="https://example.com/example.jpg",
+            filename="example.jpg",
+            extension=".jpg",
+            mime_type="image/jpeg",
+            size=100,
+        )
+        cases: list[TestCase] = [
+            TestCase(
+                name="number/int",
+                value=build_segment(1),
+            ),
+            TestCase(
+                name="number/float",
+                value=build_segment(1.0),
+            ),
+            TestCase(
+                name="string",
+                value=build_segment("a"),
+            ),
+            TestCase(
+                name="object",
+                value=build_segment({}),
+            ),
+            TestCase(
+                name="file",
+                value=build_segment(test_file),
+            ),
+            TestCase(
+                name="array[any]",
+                value=build_segment([1, "a"]),
+            ),
+            TestCase(
+                name="array[string]",
+                value=build_segment(["a", "b"]),
+            ),
+            TestCase(
+                name="array[number]/int",
+                value=build_segment([1, 2]),
+            ),
+            TestCase(
+                name="array[number]/float",
+                value=build_segment([1.0, 2.0]),
+            ),
+            TestCase(
+                name="array[number]/mixed",
+                value=build_segment([1, 2.0]),
+            ),
+            TestCase(
+                name="array[object]",
+                value=build_segment([{}, {"a": 1}]),
+            ),
+            TestCase(
+                name="none",
+                value=build_segment(None),
+            ),
+        ]
+
+        for idx, c in enumerate(cases, 1):
+            fail_msg = f"test case {c.name} failed, index={idx}"
+            draft_var = WorkflowDraftVariable()
+            draft_var.set_value(c.value)
+            assert c.value == draft_var.get_value(), fail_msg
+
+    def test_file_variable_preserves_all_fields(self):
+        """Test that File type variables preserve all fields during encoding/decoding."""
+        tenant_id = "test_tenant_id"
+
+        # Create a File with specific field values
+        test_file = File(
+            id="test_file_id",
+            tenant_id=tenant_id,
+            type=FileType.IMAGE,
+            transfer_method=FileTransferMethod.REMOTE_URL,
+            remote_url="https://example.com/test.jpg",
+            filename="test.jpg",
+            extension=".jpg",
+            mime_type="image/jpeg",
+            size=12345,  # Specific size to test preservation
+            storage_key="test_storage_key",
+        )
+
+        # Create a FileSegment and WorkflowDraftVariable
+        file_segment = build_segment(test_file)
+        draft_var = WorkflowDraftVariable()
+        draft_var.set_value(file_segment)
+
+        # Retrieve the value and verify all fields are preserved
+        retrieved_segment = draft_var.get_value()
+        retrieved_file = retrieved_segment.value
+
+        # Verify all important fields are preserved
+        assert retrieved_file.id == test_file.id
+        assert retrieved_file.tenant_id == test_file.tenant_id
+        assert retrieved_file.type == test_file.type
+        assert retrieved_file.transfer_method == test_file.transfer_method
+        assert retrieved_file.remote_url == test_file.remote_url
+        assert retrieved_file.filename == test_file.filename
+        assert retrieved_file.extension == test_file.extension
+        assert retrieved_file.mime_type == test_file.mime_type
+        assert retrieved_file.size == test_file.size  # This was the main issue being fixed
+        # Note: storage_key is not serialized in model_dump() so it won't be preserved
+
+        # Verify the segments have the same type and the important fields match
+        assert file_segment.value_type == retrieved_segment.value_type
+
+    def test_get_and_set_value(self):
+        draft_var = WorkflowDraftVariable()
+        int_var = IntegerSegment(value=1)
+        draft_var.set_value(int_var)
+        value = draft_var.get_value()
+        assert value == int_var