Implement fail-safe fallback and PII redaction on guard failures - #1821

Why:

When text extraction from LLM responses fails, the @guard decorator previously returned raw payloads. This silent failure mechanism bypassed downstream schema validations and global PII/PHI redaction filters, creating a significant risk of leaking unredacted patient data and violating HIPAA compliance.

To eliminate this vulnerability, we have introduced a robust, fail-safe fallback mechanism. Our architecture enforces a strict distinction based on the execution mode:

• fail-fast mode: Prioritizes absolute data safety by immediately halting the workflow and raising a validation exception, ensuring unvalidated clinical data never enters downstream databases.
• warning mode: Facilitates safe system monitoring and debugging by stringifying the unrecognized payload, applying high-performance regex-based redaction for sensitive identifiers (SSNs, emails, phone numbers, dates), and returning the safe string.
• Auditability: To meet strict compliance standards, all extraction failures must be cryptographically signed and archived so that the system state during failure is fully auditable and tamper-proof.

Key Decisions & Rationale:

1. Performance over Complexity: Fallback redaction in warning mode is executed using a optimized regex parser rather than dynamic JSON schema parsers. This ensures processing remains well under the 50ms latency budget.
2. Cryptographic Accountability: Audit logs are serialized and coupled with an HMAC-SHA256 signature (.sig) generated via the workspace signing key. This guarantees the integrity of the audit trail for compliance verification.
3. Fail-Safe Strictness: Any text extraction failure (ValueError during extract_text) is treated as a security event, triggering immediate interception before any logging or output returning occurs.

What:

• Modified @guard Decorator (promptops/promptops/guard.py):
  • Wrapped LLM text extraction in a try-except block to catch ValueError.
  • fail_fast Behavior: Intercepts failures, records a signed audit log, and raises ProomptsValidationError.
  • warning Behavior: Converts raw payloads to standard strings, runs regex redaction targeting SSNs, emails, phone numbers, and dates, writes a signed audit log, and returns the sanitized string.
• Added Cryptographic Auditing:
  • Integrated get_signing_key() and get_workspace_audit_dir() to write companion .json (payload state) and .sig (HMAC signature) files to the workspace audit trail on failure.
• Expanded Test Coverage (tests/test_guard.py):
  • Added test cases verifying validation exceptions are raised under fail_fast.
  • Added test cases ensuring stringification and precise regex redaction of sensitive identifiers under warning.
  • Verified audit files and signatures are correctly generated and verified.