refactor(github): extract doRequestCore, address review feedback

- MAJOR: Extract doRequestCore to eliminate doRequest/doRequestWithBody
  duplication. Both now delegate to a shared implementation with the
  retry/backoff logic in a single place.

- MINOR: Replace custom containsStr/containsSubstring helpers with
  strings.Contains in review_test.go.

- MINOR: Use http.Method* constants (MethodPost, MethodDelete, MethodPut)
  in review.go for consistency with doGet.

- MINOR: Remove redundant APPROVED/DISMISSED cases from
  translateGitHubReviewState that were identical to the default passthrough.

- NIT: Clarify DeleteReview comment about COMMENTED being a GitHub API
  state name.

- DismissReview Event field verified as required by GitHub API docs;
  kept as-is.

github/review_test.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 
@@ -153,7 +154,7 @@ func TestPostReview_MalformedResponse(t *testing.T) {
 	if err == nil {
 		t.Fatal("expected error for malformed response")
 	}
-	if !containsStr(err.Error(), "parse review response") {
+	if !strings.Contains(err.Error(), "parse review response") {
 		t.Errorf("expected parse error, got: %v", err)
 	}
 }
@@ -379,16 +380,4 @@ func TestTranslateGitHubReviewState(t *testing.T) {
 	}
 }
 
-// containsStr is a test helper for checking error messages.
-func containsStr(s, substr string) bool {
-	return len(s) >= len(substr) && (s == substr || len(s) > 0 && containsSubstring(s, substr))
-}
 
-func containsSubstring(s, sub string) bool {
-	for i := 0; i <= len(s)-len(sub); i++ {
-		if s[i:i+len(sub)] == sub {
-			return true
-		}
-	}
-	return false
-}