fix(core/group): show expand-collapse icon

[danielleroux]

💡 What is the current behavior?

GitHub Issue Number: Closes #1206

🆕 What is the new behavior?

Keep <slot> rendered to ensure items check can be fulfilled

🏁 Checklist

A pull request can only be merged if all of these conditions are met (where applicable):

• [ ] 🦮 Accessibility (a11y) features were implemented
• [ ] 🗺️ Internationalization (i18n) - no hard coded strings
• [ ] 📲 Responsiveness - components handle viewport changes and content overflow gracefully
• [ ] 📄 Documentation was reviewed/updated (pnpm run docs)
• [x] 🧪 Unit tests were added/updated and pass (pnpm test)
• [x] 📸 Visual regression tests were added/updated and pass (Guide)
• [x] 🧐 Static code analysis passes (pnpm lint)
• [x] 🏗️ Successful compilation (pnpm build, changes pushed)

👨‍💻 Help & support

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 5e9cbf190a6dbbb60b7da2b1fcd191e07a40b1a7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

| Name | Type | | ------------------- | ----- | | @siemens/ix | Patch | | @siemens/ix-angular | Patch | | @siemens/ix-react | Patch | | @siemens/ix-vue | Patch |

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