Refactor duplicated graph visualization code in html report

[saurabhsharma2u]

This change addresses the code health issue of duplicated graph visualization code.

The generateHtml function in plugins/core/src/report/html.ts previously contained a duplicated and simpler implementation of the graph visualization. This has been refactored to use the more robust SITEGRAPH_HTML template from plugins/core/src/report/sitegraph_template.ts.

Key changes:

1. generateHtml now injects graphData and metrics into SITEGRAPH_HTML instead of generating its own HTML.
2. graphData is sanitized (HTML content removed) before injection to keep the file size manageable.
3. sitegraph_template.ts was updated to display session crawl statistics (fetched/cached pages), ensuring no regression in the information provided by the report.
4. Tests in html_report.test.ts were updated to verify the correct data injection.

This consolidation reduces maintenance overhead and provides a better visualization experience for the HTML report.

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PR created automatically by Jules for task 11969879025056150495 started by @saurabhsharma2u

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