fix: disable duplicate trigger nodes in workflow builder (#402)

* 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

* style: format test_text_chat_session_service.py with ruff

* chore: retrigger CI checks

* fix(workflow): enforce node instance constraints

---------

Co-authored-by: Abhishek Kumar <abhishek@a6k.me>

api/services/integrations/tuner/node.py

@@ -40,10 +40,7 @@ from api.services.workflow.node_specs.model_spec import (
         )
     ],
     graph_constraints=GraphConstraints(
-        min_incoming=0,
-        max_incoming=0,
-        min_outgoing=0,
-        max_outgoing=0,
+        min_incoming=0, max_incoming=0, min_outgoing=0, max_outgoing=0, max_instances=1
     ),
     property_order=(
         "name",