Fix: don't use owner name as source_project_id for cross-repo MRs

sourceProjectIdentifier() was falling back to source.owner (a
username/group string) when source.repository was absent.  This sent
a bogus --raw-field source_project_id=<username> to the GitLab API,
which expects a numeric project ID.

Only use source.repository (a proper path_with_namespace) for the
source_project_id parameter; when only an owner is known, omit it
entirely and let the endpoint default to the current project.

Author: cursoragent

apps/server/src/sourceControl/GitLabCli.ts

@@ -206,7 +206,7 @@ function sourceRefName(input: {
 }
 
 function sourceProjectIdentifier(source: SourceControlRefSelector | undefined): string | null {
-  return source?.repository ?? source?.owner ?? null;
+  return source?.repository ?? null;
 }
 
 function toSummaryWithOptionalUpdatedAt(