perf(tree_view): memoize EuiTreeView styles with useEuiStylesMemoizer

[as-marin]

• What: Switched euiTreeViewStyles to use the memoized styles pattern.
• Why: Fixes [Emotion] Memoize EuiTreeView styles #10169 (Part of [Meta] [Emotion] Memoize remaining generated styles #10147). This aligns the component with the standard for performance optimization using useEuiMemoizedStyles.
• How: Updated tree_view.tsx to use stylesMemoizer and verified that euiTreeViewStyles is a static theme-based generator. Dynamic props (like display) are kept outside the memoized function to maintain reactivity.

### API Changes
```markdown
No changes to the public EUI API.

Screenshots

No visual changes.

Impact Assessment

- [x] 🔴 **Breaking changes** — None.
- [x] 💅 **Visual changes** — None.
- [x] 🧪 **Test impact** — None. Unit tests passed in both React 17 and 18.
- [x] 🔧 **Hard to integrate** — No.

**Impact level:** 🟢 None

Release Readiness

- [x] **Documentation:** N/A
- [x] **Figma:** N/A
- [x] **Migration guide:** N/A
- [x] **Adoption plan:** N/A

QA instructions for reviewer

- Open the TreeView component in Storybook.
- Confirm that the component renders correctly in both Light and Dark modes.
- Verify that there are no console errors or rendering issues.

Checklist before marking Ready for Review

- [x] Filled out all sections above
- [x] **QA:** Tested [light/dark modes, high contrast](https://devtoolstips.org/tips/en/emulate-forced-colors/), mobile, Chrome/Safari/Edge/Firefox, keyboard-only, screen reader
- [x] **QA:** Tested in [CodeSandbox](https://codesandbox.io/) and [Kibana](https://github.com/elastic/eui/blob/main/wiki/contributing-to-eui/testing/testing-in-kibana.md)
- [x] **QA:** Tested docs changes
- [x] **Tests:** Added/updated [Jest](https://github.com/elastic/eui/blob/main/wiki/contributing-to-eui/testing/unit-testing.md), [Cypress](https://github.com/elastic/eui/blob/main/wiki/contributing-to-eui/testing/cypress-testing.md), and [VRT](https://github.com/elastic/eui/blob/main/wiki/contributing-to-eui/testing/visual-regression-testing.md)
- [x] **Changelog:** Added [changelog entry](https://github.com/elastic/eui/blob/main/wiki/contributing-to-eui/documenting/changelogs.md)
- [x] **Breaking changes:** Added `breaking change` label (if applicable)

[github-actions]

👋 Since this is a community submitted pull request, a Buildkite build has not been started automatically. Would an Elastic organization member please verify the contents of this pull request and kick off a build manually?

[angeles-mzb]

buildkite test this

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

• Buildkite Build
• Commit: d364efd

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

• Buildkite Build
• Commit: d364efd
• Documentation website
• Storybook
• ✅ Visual regression tests passed

[angeles-mzb]

As a side note, before starting work, it's a good idea to comment on the issue you'd like to tackle so other community contributors see the issue is already being worked on.

Thanks for contributing @as-marin!