feat: support custom descriptions for subgraph tooltips (#9003)

## Summary

Adds support for custom descriptions on subgraph nodes that display as
tooltips when hovering.

## Changes

- Add optional `description` field to `ExportedSubgraph` interface and
`Subgraph` class
- Use description with fallback to default string in
`subgraphService.createNodeDef()`
- Add `description` to `SubgraphDefinitionBase` interface and Zod schema
for validation

## Review Focus

- Backwards compatibility: undefined description falls back to `Subgraph
node for ${name}`
- Serialization pattern: conditional spread `...(this.description && {
description })`

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9003-feat-support-custom-descriptions-for-subgraph-tooltips-30d6d73d36508129bd75c77eb1c31cfb)
by [Unito](https://www.unito.io)

src/platform/workflow/validation/schemas/workflowSchema.ts

@@ -396,6 +396,8 @@ interface SubgraphDefinitionBase<
   id: string
   revision: number
   name: string
+  /** Optional description shown as tooltip when hovering over the subgraph node. */
+  description?: string
   category?: string
   /** Custom metadata for the subgraph (description, searchAliases, etc.) */
   extra?: T extends ComfyWorkflow1BaseInput
@@ -432,6 +434,8 @@ const zSubgraphDefinition = zComfyWorkflow1
     id: z.string().uuid(),
     revision: z.number(),
     name: z.string(),
+    /** Optional description shown as tooltip when hovering over the subgraph node. */
+    description: z.string().optional(),
     category: z.string().optional(),
     inputNode: zExportedSubgraphIONode,
     outputNode: zExportedSubgraphIONode,