Harden agentic site creation

[bcotrim]

Related issues

• N/A — PR 2 of the approved revised split for Agentic UI: Site Creation #3941

How AI was used in this PR

Codex helped compare the original umbrella PR with current trunk, implement the isolated reliability changes, and add focused tests. The resulting scope and behavior were manually reviewed and verified.

Proposed Changes

• Make site creation resilient when suggested values arrive asynchronously, while preserving fields the user already edited.
• Keep generated paths current across rapid name changes, preserve manual paths, surface path failures, and prevent submission with path errors.
• Offer supported WordPress versions when available and fall back safely to the default version for unsupported or unavailable suggestions.
• Add reusable available-name/path lookup support so later onboarding flows can avoid site-name and directory collisions without expanding this PR into blueprint, import, or connect orchestration.

Testing Instructions

• Run npx eslint --fix on the modified files.
• Run npm run typecheck.
• Run npm test -- apps/ui/src packages/common/lib/tests/generate-site-name.test.ts.
• Run npm run cli:build, start studio ui, and open /onboarding/create.
• Confirm changing the site name refreshes the proposed path, a manually edited browser path is preserved, path errors block submission, and the WordPress version control contains the supported version list.
• Check the form and advanced settings in both light and dark color schemes. In Electron, also smoke-test the native folder picker with a valid folder, a path already used by Studio, and a non-empty non-WordPress folder.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?