fix(workflow): replace hardcoded trigger node logic with metadata-driven approach

- Add isStart: true to all trigger nodes (TriggerWebhook, TriggerSchedule, TriggerPlugin)
- Replace hardcoded BlockEnum checks in use-checklist.ts with metadata-driven logic
- Update trigger node tests to validate metadata instead of obsolete methods
- Add webhook URL validation to TriggerWebhook node
- Ensure backward compatibility with existing workflow configurations

This change migrates from hardcoded trigger node identification to a
centralized metadata-driven approach, improving maintainability and
consistency across the workflow system.

web/app/components/workflow/nodes/trigger-plugin/default.ts

@@ -6,6 +6,7 @@ import type { PluginTriggerNodeType } from './types'
 const metaData = genNodeMetaData({
   sort: 1,
   type: BlockEnum.TriggerPlugin,
+  isStart: true,
 })
 
 const nodeDefault: NodeDefault<PluginTriggerNodeType> = {