Fix Proxmox credential guest imports

[ZacharyZcR]

Summary

• allow credential-backed SSH imports to inherit the credential username
• keep username validation unchanged for password, key, and other SSH authentication modes
• store an empty string instead of null when the host username comes from a credential
• cover credential SSH and non-SSH import username behavior with regression tests

Verification

• npm test -- --run src/backend/tests/database/routes/host-bulk-routes.test.ts src/backend/tests/database/routes/host-normalizers.test.ts src/backend/tests/database/routes/proxmox-import-auth.test.ts src/ui/tests/components/proxmox/proxmox-import-auth.test.ts
• npm run type-check
• npm run build
• targeted ESLint and Prettier checks

Fixes Termix-SSH/Support#1195