fix: address self-review findings

- Remove dead code: findOwnReview (replaced by findAllOwnReviews)
- Check SetRetryBackoff return value in doJSONRequest tests
- Extract doWithRetry shared helper to eliminate ~100 lines of
  duplicated 429-retry/backoff/Retry-After logic between doRequest
  and doJSONRequest
- Fix import order: context before encoding/json (goimports)
- Add slog.Warn when ListReviews hits maxReviewPages limit

github/client_test.go

@@ -609,7 +609,9 @@ func TestDoJSONRequest_429Retry(t *testing.T) {
 	defer ts.Close()
 
 	c := NewClient("token", ts.URL, AllowInsecureHTTP())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	body, err := c.doJSONRequest(context.Background(), http.MethodPost, ts.URL+"/test", map[string]string{"key": "val"})
 	if err != nil {
@@ -631,7 +633,9 @@ func TestDoJSONRequest_429ExhaustsRetries(t *testing.T) {
 	defer ts.Close()
 
 	c := NewClient("token", ts.URL, AllowInsecureHTTP())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	_, err := c.doJSONRequest(context.Background(), http.MethodPost, ts.URL+"/test", map[string]string{"key": "val"})
 	if err == nil {