fix: respect custom filename during PDF generation

[Jannik-cmyk]

Summary

• use "outFilename" as the source HTML file during PDF generation
• derive the PDF filename from the configured HTML filename
• construct cross-platform-safe file URLs
• always close Puppeteer after success or failure
• add focused regression tests for default, custom and failing paths

Root cause

The HTML template writes the generated documentation to
"params.outFilename || 'index.html'", but the PDF hook always read
"index.html" and wrote "index.pdf". PDF generation therefore failed or
generated the wrong output whenever "outFilename" was customized.

Validation

• "node --check hooks/99_generatePdf.js"
• "node --check test/hooks/99_generatePdf.test.js"
• direct mocked Node harness covering filename derivation, URL encoding,
  browser cleanup and failure paths
• "git diff --check"
• "npm run lint"

Fixes #826

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[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud