[Emotion] Memoize EuiHeaderSection styles

[Anshuman-Prakash]

Summary

Fixes #10156. Part of #10147.

Memoize EuiHeaderSection styles with useEuiMemoizedStyles(euiHeaderSectionStyles) instead of calling the generator on every render. The generator takes no arguments, so it is already a static function of the theme; the grow/side style selection stays in the component.

API Changes

None.

Screenshots

Not applicable: CSS definitions are unchanged, and all existing header snapshots pass without updates on React 17 and React 18.

Impact Assessment

Impact level: 🟢 Low — performance-only change to reuse generated styles across renders. No expected visual, behavioral, public API, or snapshot changes.

Release Readiness

• Documentation, Figma, migration guide, and adoption plan: not applicable to this internal optimization.
• No changelog, as requested in the issue; skip-changelog applies.

QA instructions for reviewer

From packages/eui:

• yarn test-unit src/components/header --runInBand — passed: 62 tests, 38 snapshots on React 18.
• yarn test-unit --react-version=17 src/components/header --runInBand — passed: 62 tests, 38 snapshots on React 17.
• eslint src/components/header/header_section/ and yarn tsc --noEmit — no errors.

[github-actions]

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