Fix discovery drain timeout and session tokens

[dkropachev]

Summary

• bound non-2xx /localnodes response draining and destroy/cancel bodies that do not finish
• drop AWS session tokens before signing Alternator requests because Alternator does not consume them
• add regression coverage and update docs

Testing

• npm run typecheck
• npm run lint
• npm test
• npm run build
• npm run test:integration (opt-in gate only: 1 passed, 55 skipped)
• npm audit --omit=dev --audit-level=moderate

Full Docker integration suite was not run.