Handle non-JSON OAuth errors

[Delcado19]

Handle non-JSON OAuth and API error responses

Fixes #1318.

Summary

• Adds safe response parsing helpers for JSON, text, and empty response bodies.
• Avoids Unexpected token ... crashes when OAuth/provider endpoints return plain-text or HTML error responses.
• Updates OAuth modal flows to surface useful error messages instead of failing while parsing the error response.

Validation

npx vitest run --config ./vitest.config.js --reporter=verbose api-response.test.js xai-oauth-service.test.js
-> 8 passed

git diff --check origin/master..HEAD
-> passed

npm run build
-> passed

Notes

This PR is intentionally scoped to shared response parsing and OAuth modal error handling.