Return guide get payload after study guide wait

[yupeng-dev]

Summary

• Change study guide wait so a successful wait fetches /v1/studies/:id/discussion-guide and returns that guide-get payload.
• Keeps polling/failure/timeout behavior intact; only the success payload changes.

PR scope

• Source-only change; no spec or unit-test files are included.

Validation

• npm ci
• npm run build
• npm run typecheck
• env -u COOKIY_SERVER_URL npm test
• git diff --check

Manual note

• The env-unset test command avoids inheriting a local COOKIY_SERVER_URL override into the existing test suite.