fix: disable duplicate trigger nodes in workflow builder

AddNodePanel: disable trigger buttons and show tooltip when a trigger
already exists on the canvas, using bySpecName to identify trigger-
category specs from the live node list.
useWorkflowState: preflight in saveWorkflow rejects saves with multiple
trigger nodes via a sonner toast before the network request is made.
text_chat_session_service: include the original exception message in
TextChatSessionExecutionError so the HTTP 500 detail surfaces the root
cause without DB inspection.

Closes #378

api/services/workflow/text_chat_session_service.py

@@ -207,7 +207,7 @@ async def execute_pending_text_chat_turn(
             error_message=str(e),
         )
         raise TextChatSessionExecutionError(
-            "Failed to execute text chat assistant turn"
+            f"Failed to execute text chat assistant turn: {e}"
         ) from e
 
     completed_session_data = normalize_text_chat_session_data(text_session.session_data)

api/tests/test_text_chat_session_service.py

@@ -1,4 +1,4 @@
-from unittest.mock import AsyncMock
+from unittest.mock import AsyncMock, patch
 
 import pytest
 
@@ -9,6 +9,7 @@ from api.services.workflow.text_chat_session_service import (
     TextChatTurnNotFoundError,
     _reload_text_chat_session,
     build_pending_text_chat_turn,
+    execute_pending_text_chat_turn,
     truncate_text_chat_future_turns,
     validate_text_chat_turn_cursor,
 )
@@ -77,6 +78,31 @@ async def test_reload_text_chat_session_uses_run_id_to_resolve_organization(
     get_text_session.assert_awaited_once_with(123, organization_id=77)
 
 
+@pytest.mark.asyncio
+async def test_execute_pending_turn_surfaces_original_exception_message(monkeypatch):
+    session = WorkflowRunTextSessionModel(workflow_run_id=42)
+    session.session_data = {"turns": [{"id": "turn-1", "status": "pending"}], "cursor_turn_id": "turn-1"}
+    session.checkpoint = None
+
+    monkeypatch.setattr(
+        text_chat_session_service,
+        "execute_text_chat_pending_turn",
+        AsyncMock(side_effect=RuntimeError("Workflow has 2 start nodes")),
+    )
+    monkeypatch.setattr(
+        text_chat_session_service,
+        "_mark_pending_turn_failed",
+        AsyncMock(),
+    )
+
+    with pytest.raises(TextChatSessionExecutionError, match="Workflow has 2 start nodes"):
+        await execute_pending_text_chat_turn(
+            workflow_id=1,
+            run_id=42,
+            text_session=session,
+        )
+
+
 @pytest.mark.asyncio
 async def test_reload_text_chat_session_raises_when_run_organization_is_missing(
     monkeypatch,