fix: return distinct not-found errors for each principal type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: whoAbhishekSah

internal/api/v1beta1connect/project.go

@@ -384,8 +384,12 @@ func (h *ConnectHandler) SetProjectMemberRole(ctx context.Context, request *conn
 		switch {
 		case errors.Is(err, project.ErrNotExist):
 			return nil, connect.NewError(connect.CodeNotFound, ErrNotFound)
-		case errors.Is(err, user.ErrNotExist), errors.Is(err, serviceuser.ErrNotExist), errors.Is(err, group.ErrNotExist):
-			return nil, connect.NewError(connect.CodeNotFound, ErrNotFound)
+		case errors.Is(err, user.ErrNotExist):
+			return nil, connect.NewError(connect.CodeNotFound, ErrUserNotExist)
+		case errors.Is(err, serviceuser.ErrNotExist):
+			return nil, connect.NewError(connect.CodeNotFound, ErrServiceUserNotFound)
+		case errors.Is(err, group.ErrNotExist):
+			return nil, connect.NewError(connect.CodeNotFound, ErrGroupNotFound)
 		case errors.Is(err, project.ErrNotOrgMember):
 			return nil, connect.NewError(connect.CodeFailedPrecondition, ErrNotMember)
 		case errors.Is(err, role.ErrNotExist):