address review feedback: wrap ErrCannotDeleteSubmittedReview, fix nits

- Wrap ErrCannotDeleteSubmittedReview with operation context via fmt.Errorf
  so callers get both sentinel identity and context (MINOR fix)
- Combine double iteration in PostReview into single loop (NIT)
- Remove extra trailing blank line in review_test.go (NIT)
- Clarify translateGitHubReviewState comment re: PENDING state (NIT)
- Update requestOptions.bodyFn comment to mention DELETE-with-body (NIT)

github/client.go

@@ -196,7 +196,8 @@ func (c *Client) SetRetryBackoff(d []time.Duration) error {
 // requestOptions holds per-request configuration for doRequestCore.
 type requestOptions struct {
 	// bodyFn returns a fresh io.Reader for the request body on each attempt.
-	// Must be non-nil for requests that carry a body (POST, PUT, PATCH).
+	// Must be non-nil for any request that carries a body (POST, PUT, PATCH,
+	// or DELETE when a body is required by the API).
 	// Returning a fresh reader on each call allows retries to re-send the body.
 	bodyFn func() io.Reader