feat(diff): Enable commenting on expanded lines in PR diffs (#35662)

Fixes #32257 
/claim #32257

Implemented commenting on unchanged lines in Pull Request diffs, lines
are accessed by expanding the diff preview. Comments also appear in the
"Files Changed" tab on the unchanged lines where they were placed.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/pull/review.go

@@ -22,6 +22,7 @@ import (
 	"code.gitea.io/gitea/modules/optional"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
+	"code.gitea.io/gitea/services/gitdiff"
 	notify_service "code.gitea.io/gitea/services/notify"
 )
 
@@ -283,6 +284,15 @@ func createCodeComment(ctx context.Context, doer *user_model.User, repo *repo_mo
 			log.Error("Error whilst generating patch: %v", err)
 			return nil, err
 		}
+
+		// If patch is still empty (unchanged line), generate code context
+		if patch == "" && commitID != "" {
+			patch, err = gitdiff.GeneratePatchForUnchangedLine(gitRepo, commitID, treePath, line, setting.UI.CodeCommentLines)
+			if err != nil {
+				// Log the error but don't fail comment creation
+				log.Debug("Unable to generate patch for unchanged line (file=%s, line=%d, commit=%s): %v", treePath, line, commitID, err)
+			}
+		}
 	}
 	return issues_model.CreateComment(ctx, &issues_model.CreateCommentOptions{
 		Type:        issues_model.CommentTypeCode,