Fix websec self-scan findings and false positives

[google-labs-jules]

This PR fixes several true and false positive findings flagged by websec run ..

• Fixes the true positive docker-no-healthcheck finding by explicitly declaring a HEALTHCHECK in Dockerfile.
• Fixes the false positive llm-guardrail-fail-open logic in src/websec_validator/extractors/llm_security.py by removing the overly aggressive greedy match (\w*) which flagged unrelated terms like Scanner and scan_dir. A regression test was added to verify it explicitly.
• Suppresses false positives generated by other internal extractors (such as llm_security.py, authz.py, crypto_usage.py, transport_security.py) and findings.py in .websec-ignore with justifications, as they hold attack definitions explicitly.

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PR created automatically by Jules for task 17858821003781577430 started by @raccioly

[google-labs-jules]

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