fix(pr-review): post a COMMENT review when GitHub forbids self-review

A pr-reviewer review of an org-authored PR never reached GitHub. The agent
side ran correctly (claim → read-only diff → review → post_pr_review →
completed + CEO notify), but the GitHub publish 422'd with "Can not request
changes on your own pull request": the PR was authored by the same account
that owns the project PAT. post_pr_review posts best-effort after the DB
transition, so the failure was logged and swallowed — the task completed and
the CEO was notified "reviewed" while the PR showed no review.

GitHub forbids APPROVE / REQUEST_CHANGES on your own PR but DOES allow a
plain COMMENT review. The org's internal PRs (and any PR the PAT owner
opened) hit this. Retry once as a COMMENT review on the self-review 422 so
the review actually lands; the verdict is already stated in the body. The
external/fork-PR path (different author) is unchanged — REQUEST_CHANGES
succeeds there and the fallback never fires.

Tests: self-review 422 downgrades to COMMENT and returns the COMMENT result;
a failing COMMENT retry still surfaces GitError with no infinite loop; the
existing non-self 422 still raises.

tests/unit/services/test_git_post_pr_review.py

@@ -92,3 +92,58 @@ async def test_post_pr_review_raises_on_github_error() -> None:
         pytest.raises(GitError),
     ):
         await svc.post_pr_review("roboco", _PR, "body")
+
+
+@pytest.mark.asyncio
+async def test_post_pr_review_self_review_falls_back_to_comment() -> None:
+    """A 422 'own pull request' downgrades REQUEST_CHANGES to a COMMENT review."""
+    svc = _service()
+    _bind(svc, "_token_for_project", AsyncMock(return_value="tok"))
+    client = MagicMock()
+    client.__aenter__ = AsyncMock(return_value=client)
+    client.__aexit__ = AsyncMock(return_value=False)
+    # First POST (REQUEST_CHANGES) 422s as a self-review; the COMMENT retry wins.
+    client.post = AsyncMock(
+        side_effect=[
+            _resp(
+                status_code=422,
+                text='{"errors":["Review Can not request changes on your own '
+                'pull request"]}',
+            ),
+            _resp(status_code=200, json_payload={"id": 9, "state": "COMMENTED"}),
+        ]
+    )
+    with (
+        _patch_project(),
+        patch("roboco.services.git.httpx.AsyncClient", return_value=client),
+    ):
+        out = await svc.post_pr_review("roboco", _PR, "Please fix X.")
+    assert client.post.await_count == 2  # noqa: PLR2004
+    first, second = client.post.await_args_list
+    assert first.kwargs["json"]["event"] == "REQUEST_CHANGES"
+    assert second.kwargs["json"]["event"] == "COMMENT"
+    assert second.kwargs["json"]["body"] == "Please fix X."
+    assert out["state"] == "COMMENTED"
+
+
+@pytest.mark.asyncio
+async def test_post_pr_review_self_review_comment_failure_raises() -> None:
+    """If the COMMENT retry also fails, the error surfaces (no infinite loop)."""
+    svc = _service()
+    _bind(svc, "_token_for_project", AsyncMock(return_value="tok"))
+    client = MagicMock()
+    client.__aenter__ = AsyncMock(return_value=client)
+    client.__aexit__ = AsyncMock(return_value=False)
+    client.post = AsyncMock(
+        side_effect=[
+            _resp(status_code=422, text='["...on your own pull request"]'),
+            _resp(status_code=500, text="boom"),
+        ]
+    )
+    with (
+        _patch_project(),
+        patch("roboco.services.git.httpx.AsyncClient", return_value=client),
+        pytest.raises(GitError),
+    ):
+        await svc.post_pr_review("roboco", _PR, "body")
+    assert client.post.await_count == 2  # noqa: PLR2004