fix: Properly hide separator in Menu when next node is loader/null

[LFDanLu]

From testing
Fixes the bug where the separator was appearing after the last section since we always render a loading node in S2
Also adds a set width to the async loading menu docs example so it doesn't resize as items are loader

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at CLAUDE.md.

📝 Test Instructions:

See chromatic. Test the menus with sections/combobox with sections/picker with sections and make sure no divider renders after the last item

🧢 Your Project:

RSP

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[rspbot]

Agent Skills Changes

Modified (10)

• react-aria/agent-skills/index.json $\color{green}{+1}$ $\color{red}{-1}$
• react-aria/agent-skills/react-aria.tar.gz
• react-aria/agent-skills/react-aria/references/components/Separator/useSeparator.md $\color{green}{+1}$ $\color{red}{-1}$
• s2/agent-skills/index.json $\color{green}{+3}$ $\color{red}{-3}$
• s2/agent-skills/migrate-react-spectrum-v3-to-s2.tar.gz
• s2/agent-skills/react-spectrum-s2.tar.gz
• s2/agent-skills/react-spectrum-s2/references/components/Menu.md $\color{green}{+2}$
• s2/agent-skills/react-spectrum-s2/references/guides/ai-components.md $\color{red}{-17}$
• s2/agent-skills/react-spectrum-s2/references/react-aria/components/Separator/useSeparator.md $\color{green}{+1}$ $\color{red}{-1}$
• s2/agent-skills/spectrum-audit.tar.gz

Install

React Spectrum S2:

npx skills add https://d1pzu54gtk2aed.cloudfront.net/pr/705bbd3eea955f52bf3fcc278e31a9af00590316/

React Aria:

npx skills add https://d5iwopk28bdhl.cloudfront.net/pr/705bbd3eea955f52bf3fcc278e31a9af00590316/