Remove the source handle for leaf nodes (#10057)

- Do not render a source handle for the leaf nodes
- Upgrade the `@xyflow/react` library

| Before | After |
|--------|--------|
| ![CleanShot 2025-02-06 at 16 21
08@2x](https://github.com/user-attachments/assets/42b7d11b-76bf-43b9-ba91-8d0c5c2f1792)
| ![CleanShot 2025-02-06 at 16 21
24@2x](https://github.com/user-attachments/assets/ac94aa32-45ad-4462-8db9-0078d6252ea4)
|

## Other options considered

React Flow exposes a hook to get the connections of the current node. I
tried to use this hook – which makes things way simpler – but I couldn't
find a way to make it work in Storybook. I had two options: 1. Set up
React Flow to render the nodes properly, 2. Mock the hook in Storybook.

The first option was hard to achieve as the `<Reactflow />` component
renders a whole flow, and it doesn't play well with the idea of
rendering a single node in a story.

The second option seemed overkill as mocking modules with Storybook is
not straightforward. See
https://storybook.js.org/docs/writing-stories/mocking-data-and-modules/mocking-modules.

I chose to keep the initial version of my code, written before I spot a
function simplifying the code. We can give it a look another time.

package.json

@@ -62,7 +62,7 @@
     "@types/nodemailer": "^6.4.14",
     "@types/passport-microsoft": "^1.0.3",
     "@wyw-in-js/vite": "^0.5.3",
-    "@xyflow/react": "^12.3.5",
+    "@xyflow/react": "^12.4.2",
     "add": "^2.0.6",
     "addressparser": "^1.0.1",
     "afterframe": "^1.0.2",