feat: add PDF export support to CLI (#1734)

[Harini-2811]

Description

Added PDF export support to the SecuScan CLI. Previously, the --format flag supported sarif, json, csv, html, and console but not pdf. This change adds pdf as a valid format option in the CLI, reusing the existing generate_pdf_report() logic from ReportGenerator and the build_report_filename() helper already used by the API route.

Related Issues

Closes #1734

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

1. Run a scan via CLI with explicit output path:

   python -m backend.secuscan.cli scan <target> --plugin <plugin_id> --format pdf -o report.pdf

2. Verify the PDF is saved at the specified path.
3. Open the PDF and confirm it includes scan summary, severity breakdown, and findings.
4. Run without --output to confirm auto-generated filename works:

   python -m backend.secuscan.cli scan <target> --plugin <plugin_id> --format pdf

5. Confirm existing formats (html, json, csv, sarif, console) are unaffected.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[utksh1]

Thanks for adding the PDF option. Please add focused regression coverage before this is merge-ready: a CLI test (or equivalent unit test around the scan export path) should verify that --format pdf calls the PDF report generator, writes bytes to the selected output path/default filename, and handles RuntimeError cleanly. Right now the new behavior is untested, so this could regress silently.