Upload sample-reports and standardize on Python 3.12

[agasthik]

Description of changes:

Summary

Two related housekeeping changes to the sample reports and project metadata:

1. Anonymize AWS account IDs in sample reports — real 12-digit account IDs in the committed sample HTML
   reports are replaced with well-known AWS documentation example account IDs (e.g. 111122223333,
   123456789012)
2. Standardize on Python 3.12 — the README and CI previously gave conflicting Python versions (badge
   said 3.11+, prerequisites said 3.12+, CI tested both). All references now consistently target 3.12, matching
   the Lambda runtime already pinned to python3.12 in both SAM templates.

Changes

Account ID anonymization

• sample-reports/scripts/capture_screenshots.py now runs an anonymize_account_ids() step before
  capturing screenshots. It:
  • Scans all sample reports first, collects every distinct 12-digit account ID, and builds one
    shared, deterministic mapping so an account appearing in multiple reports maps to the same placeholder
    everywhere.
  • Uses a word-boundary regex (\b\d{12}\b) so digit runs embedded in longer numbers aren't touched.
  • Exits with an error if there are more real IDs than defined placeholders.
• Re-anonymized security_assessment_single_account.html and security_assessment_multi_account.html.
• Regenerated all four screenshots from the anonymized reports.

Python 3.12 consistency

• README.md: badge 3.11+ → 3.12, prerequisites 3.12+ → 3.12, CI table description → Python 3.12.
• .github/workflows/python-tests.yml: test matrix ["3.11", "3.12"] → ["3.12"].

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