fix(github): add DismissReview Event comment; use t.Fatalf for routing assertions

- Add comment in DismissReview explaining why the Event field is required
  by the GitHub API even though DISMISS is the only valid value (#18652).
- Change t.Errorf to t.Fatalf for method/path routing assertions in test
  handlers so failures are immediately fatal instead of silently
  continuing handler execution (#18653).

github/review.go

@@ -194,7 +194,9 @@ func (c *Client) DismissReview(ctx context.Context, owner, repo string, number i
 
 	payload := dismissReviewRequest{
 		Message: message,
-		Event:   "DISMISS",
+		// Event is required by the GitHub API for dismissal requests, even though
+		// "DISMISS" is the only valid value for this endpoint.
+		Event: "DISMISS",
 	}
 
 	data, err := json.Marshal(payload)