GitService.post_pr_review posts a single review via POST /pulls/{n}/reviews
(REQUEST_CHANGES by default; APPROVE/COMMENT supported) — the first /reviews
call in the codebase. Resolves owner/repo/token from the project slug,
authenticates as the PAT owner (Bearer), and raises GitError on any token or
GitHub failure so the calling side-effect can surface it. This is the capability
the pr_reviewer's post_pr_review verb invokes after its DB commit. httpx fully
mocked in tests (request shape, auth, error paths).