[Autocomplete] Lazily derive the initial input value

[OskarEichler]

What changed

• Derives the mount-only initial input value with a lazy state initializer instead of evaluating the useRef argument on every render.
• Avoids an otherwise unused getOptionLabel call on every production render (two calls per Strict Mode render cycle).
• No public API or rendered-value behavior changes.

useRef(expression) preserves the first result but still evaluates expression during every render. A lazy state initializer matches the existing mount-only contract.

Verification

• External render/rerender control against the base: label calls increased from 6 to 11.
• The same control after the fix: label calls increased from 6 to 9, removing the two mount-only recomputations under Strict Mode.
• pnpm test:unit run useAutocomplete --project 'node:@mui/material'
• pnpm -F @mui/material typescript
• Targeted ESLint and Prettier checks.
• Full pnpm release:build.
• Full Material Node unit suite: 179 passed / 4 skipped files; 4,712 passed / 759 skipped tests.

The browser test project was not run locally because Playwright Chromium is not installed in this checkout.

Contribution

• I have followed the contributing guide.

This change and PR description were prepared with OpenAI Codex, then self-reviewed and verified locally. No independent human review has occurred yet.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-49057--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	▼-1B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

---

Check out the code infra dashboard for more information about this PR.