refactor(workflow): add new engine and utils modules

- Add engine/ directory with core components:
  - graph_builder: workflow graph construction
  - variable_pool: variable management
  - state_manager: execution state tracking
  - event_stream_handler: event processing
  - stream_output_coordinator: streaming output control
  - result_builder: result aggregation
  - runtime_schema: runtime type definitions

- Add utils/ directory with utilities:
  - expression_evaluator: safe expression evaluation
  - template_renderer: Jinja2 template rendering

api/tests/workflow/nodes/test_start_node.py

@@ -4,11 +4,11 @@
 # @Time : 2026/2/6
 import pytest
 
+from app.core.workflow.engine.variable_pool import VariablePool
 from app.core.workflow.nodes import StartNode
 from app.core.workflow.variable.base_variable import VariableType
-from app.core.workflow.variable_pool import VariablePool
 from tests.workflow.nodes.base import (
-    simple_state, 
+    simple_state,
     simple_vairable_pool,
     TEST_EXECUTION_ID,
     TEST_WORKSPACE_ID,