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/services/subgraphService.ts

@@ -49,7 +49,7 @@ export const useSubgraphService = () => {
       output_tooltips: [],
       name: id,
       display_name: name,
-      description: `Subgraph node for ${name}`,
+      description: exportedSubgraph.description || `Subgraph node for ${name}`,
       category: 'subgraph',
       output_node: false,
       python_module: 'nodes'