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

[Copilot]

Pull request overview

Replaces GraphQL pull-request draft transitions with a shared REST implementation.

Changes:

• Adds REST draft-state lookup, no-op handling, and transitions.
• Reuses the helper in general and granular tools.
• Expands REST success and error tests.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/github/pullrequests.go	Adds and integrates the REST helper.
pkg/github/pullrequests_granular.go	Reuses the REST helper.
pkg/github/pullrequests_test.go	Updates general-path tests.
pkg/github/granular_tools_test.go	Updates granular-path tests.
pkg/github/helper_test.go	Adds REST route test helpers.
pkg/github/projects_resolver_test.go	Removes trailing whitespace.