ci: match both `h2` and `h3` area headings in auto-labeler (#36572)
GitHub issue forms render the "Area (Required)" field label at
inconsistent heading levels — some issues get `### Area` (h3), others
get `## Area` (h2). The `auto-label-by-package` workflow's regex was
hardcoded to `### Area`, silently skipping issues with `## Area`
headings and never applying package labels (e.g.
[#2471](https://github.com/langchain-ai/deepagents/issues/2471)).