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.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"net/http"
 	"net/url"
 
 	"gitea.weiker.me/rodin/review-bot/vcs"
@@ -52,15 +53,13 @@ type dismissReviewRequest struct {
 // canonical vcs.Review.State value.
 func translateGitHubReviewState(state string) string {
 	switch state {
-	case "APPROVED":
-		return "APPROVED"
 	case "CHANGES_REQUESTED":
 		return "REQUEST_CHANGES"
 	case "COMMENTED":
 		return "COMMENT"
-	case "DISMISSED":
-		return "DISMISSED"
 	default:
+		// States like APPROVED, DISMISSED, and PENDING pass through unchanged
+		// as they already match the canonical vcs representation.
 		return state
 	}
 }
@@ -100,7 +99,7 @@ func (c *Client) PostReview(ctx context.Context, owner, repo string, number int,
 		return nil, fmt.Errorf("marshal review request: %w", err)
 	}
 
-	body, err := c.doRequestWithBody(ctx, "POST", reqURL, data)
+	body, err := c.doRequestWithBody(ctx, http.MethodPost, reqURL, data)
 	if err != nil {
 		return nil, fmt.Errorf("post review: %w", err)
 	}
@@ -150,12 +149,13 @@ func (c *Client) ListReviews(ctx context.Context, owner, repo string, number int
 
 // DeleteReview deletes a pull request review.
 // Only PENDING reviews can be deleted; attempting to delete a submitted review
-// (APPROVED, CHANGES_REQUESTED, COMMENTED) returns ErrCannotDeleteSubmittedReview.
+// (APPROVED, CHANGES_REQUESTED, or COMMENTED per GitHub API naming) returns
+// ErrCannotDeleteSubmittedReview.
 func (c *Client) DeleteReview(ctx context.Context, owner, repo string, number int, reviewID int64) error {
 	reqURL := fmt.Sprintf("%s/repos/%s/%s/pulls/%d/reviews/%d",
 		c.baseURL, url.PathEscape(owner), url.PathEscape(repo), number, reviewID)
 
-	_, err := c.doRequestWithBody(ctx, "DELETE", reqURL, nil)
+	_, err := c.doRequestWithBody(ctx, http.MethodDelete, reqURL, nil)
 	if err != nil {
 		var apiErr *APIError
 		if errors.As(err, &apiErr) && apiErr.StatusCode == 422 {
@@ -183,7 +183,7 @@ func (c *Client) DismissReview(ctx context.Context, owner, repo string, number i
 		return fmt.Errorf("marshal dismiss request: %w", err)
 	}
 
-	_, err = c.doRequestWithBody(ctx, "PUT", reqURL, data)
+	_, err = c.doRequestWithBody(ctx, http.MethodPut, reqURL, data)
 	if err != nil {
 		return fmt.Errorf("dismiss review: %w", err)
 	}