feat(api): support variable reference and substitution in Email delivery

The EmailDeliveryConfig.body now support referencing variables generated
by precedent nodes.

api/tests/unit_tests/core/workflow/nodes/human_input/test_email_delivery_config.py

@@ -1,4 +1,5 @@
 from core.workflow.nodes.human_input.entities import EmailDeliveryConfig, EmailRecipients
+from core.workflow.runtime import VariablePool
 
 
 def test_replace_url_placeholder_with_value():
@@ -23,3 +24,17 @@ def test_replace_url_placeholder_missing_value():
     result = config.body_with_url(None)
 
     assert result == "No link  available."
+
+
+def test_render_body_template_replaces_variable_values():
+    config = EmailDeliveryConfig(
+        recipients=EmailRecipients(),
+        subject="Subject",
+        body="Hello {{#node1.value#}} {{#url#}}",
+    )
+    variable_pool = VariablePool()
+    variable_pool.add(["node1", "value"], "World")
+
+    result = config.render_body_template(body=config.body, url="https://example.com", variable_pool=variable_pool)
+
+    assert result == "Hello World https://example.com"