feat: add page number support to v1 html partition (#4327)
This PR adds support for page-number when partitioning html using the v1
parser.
- Add `page_number` support to the v1 HTML parser by reading
`data-page-number `attributes from ancestor elements, consistent with v2
parser behavior
- Add `_page_number` cached property on Flow using efficient
parent-chain lookup (O(n) total vs O(n*depth) ancestor walk)
- Wire page number into all three element-creation paths: text elements,
images, and tables
- Malformed `data-page-number` values are skipped and fall back to the
nearest valid ancestor
