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/tests/test_node_specs.py

@@ -414,4 +414,9 @@ def test_to_mcp_dict_retains_authoring_signal_startcall():
     ]
 
     # graph_constraints drops its null sub-fields.
-    assert projected["graph_constraints"] == {"min_incoming": 0, "max_incoming": 0}
+    assert projected["graph_constraints"] == {
+        "min_incoming": 0,
+        "max_incoming": 0,
+        "min_instances": 1,
+        "max_instances": 1,
+    }