fix: span empty_policies diagnostic on labelValues, not policies

- Change json_span_for_quoted_literal to return Option<SourceSpan> instead of silent (0,0)
- Search for labelValues key instead of non-existent policies key in getRecord response
- Update EmptyPoliciesError to use Option<SourceSpan> with label 'labelValues is empty'
- Update all error struct spans to Option<SourceSpan> for consistency
- Regenerate empty_policies snapshot showing correct span at labelValues key

Fixes I-NEW-3: Diagnostic now points at correct location in labeler record

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>