Use REST endpoints for PR draft state transitions

[aalmanasir]

Summary

• replace GraphQL draft-state mutations with REST draft-state transitions in update_pull_request
• reuse the same REST draft-state helper in update_pull_request_draft_state
• add tests for no-op behavior and REST 403/404 error handling in both general and granular paths

Notes

• this branch removes convertPullRequestToDraft and markPullRequestReadyForReview usage from the two affected code paths
• still needs authenticated runtime smoke test for app-token behavior