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/workflow/dto.py

@@ -196,7 +196,12 @@ class _ToolDocumentRefsMixin(BaseModel):
             },
         )
     ],
-    graph_constraints=GraphConstraints(min_incoming=0, max_incoming=0),
+    graph_constraints=GraphConstraints(
+        min_incoming=0,
+        max_incoming=0,
+        min_instances=1,
+        max_instances=1,
+    ),
     property_order=(
         "name",
         "greeting_type",
@@ -539,6 +544,7 @@ class EndCallNodeData(
         max_incoming=0,
         min_outgoing=0,
         max_outgoing=0,
+        max_instances=1,
     ),
     property_order=("name", "prompt"),
     field_overrides={
@@ -597,7 +603,11 @@ class GlobalNodeData(BaseNodeData, _PromptedNodeDataMixin):
     examples=[
         NodeExample(name="default", data={"name": "Inbound Trigger", "enabled": True})
     ],
-    graph_constraints=GraphConstraints(min_incoming=0, max_incoming=0),
+    graph_constraints=GraphConstraints(
+        min_incoming=0,
+        max_incoming=0,
+        max_instances=1,
+    ),
     property_order=("name", "enabled", "trigger_path"),
     field_overrides={
         "name": {