feat: add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add Atlas Cloud to the built-in third-party provider registry
• configure its OpenAI-compatible endpoint and default nested model ID
• cover session routing through the existing third-party chat path

Verification

• npx vitest run src/utils/chatApiRoute.test.ts (2 passed)
• npx eslint src/types/settings.ts src/utils/thirdPartyApiProviders.ts src/utils/chatApiRoute.test.ts --max-warnings=0
• npx prettier --check src/types/settings.ts src/utils/thirdPartyApiProviders.ts src/utils/chatApiRoute.test.ts
• npm run build
• npm run build:api
• live model discovery returned 123 Atlas Cloud models; the default model was present and a chat request through sendOpenAICompatibleMessageNonStream returned content

Existing baseline issues

• npm run typecheck still reports the existing LanguageIcon.tsx:218 compactLabel error
• the full Vitest run still has three existing useAppEvents.test.tsx Tab-cycle failures; all three reproduce on the unmodified base commit

Risk

• the new provider is disabled by default and reuses the existing OpenAI-compatible model discovery, connection test, and chat request paths.