SPW-20453 secure API Explorer credentials and production access

[dawoodali-showpass]

Summary

• remove API key and Partner credential persistence from sessionStorage, including cleanup of values written by the previous Explorer
• block authenticated Explorer requests to Showpass production before signing or network I/O
• document the Beta/Demo-only Explorer boundary and the partner profile update contract

Validation

• npm test
• npm run build
• npx eslint src/docs-app/ui/components/api/ApiExplorer.tsx
• npx tsc -p tsconfig.app.json --noEmit
• npm run lint (reports pre-existing unrelated errors in the repository; changed file passes)

Jira: SPW-20453