Fix custom download URL filenames

[cyphercodes]

What/Why/How?

Custom downloadUrls without an explicit downloadFileName currently inherit the generated-spec fallback filename (openapi.json). This forces YAML or other custom download URLs to save as openapi.json.

This change only applies the openapi.json fallback when Redoc generates the downloadable spec itself. Custom downloadUrls now leave the filename unset unless downloadFileName is explicitly configured, so the browser can use the URL/content-disposition filename.

Reference

Fixes #2787

Tests

• npx jest src/services/__tests__/models/ApiInfo.test.ts --runInBand --coverage=false
• npm run ts-check
• npx eslint src/services/models/ApiInfo.ts src/services/__tests__/models/ApiInfo.test.ts --no-cache
• npx prettier --check src/services/models/ApiInfo.ts src/services/__tests__/models/ApiInfo.test.ts
• git diff --check

Screenshots (optional)

N/A

Check yourself

• Code is linted
• Tested
• All new/updated code is covered with tests