fix: preserve model form during config refresh

[mturac]

Summary

• avoid resetting the Create Model dialog while it remains open across provider/config prop refreshes
• keep the initial open/edit reset behavior intact
• add a regression test that rerenders with refreshed provider props and preserves typed input

Fixes #184

Checks

• pnpm --filter frontend exec vitest run src/components/settings/OpenCodeModelsEditor.test.tsx
• pnpm --filter frontend exec eslint src/components/settings/OpenCodeModelDialog.tsx src/components/settings/OpenCodeModelsEditor.test.tsx --max-warnings=0