fix: Fix AuditEvent import bug in delete_pattern + add 90%+ test coverage

- Fixed bug in secure_memdocs.py line 1136 where delete_pattern incorrectly
  referenced self.audit_logger.AuditEvent instead of imported AuditEvent class
- Added test_secure_memdocs_extended.py with 47 tests covering encryption,
  error handling, classification edge cases, and delete functionality
- Added test_audit_logger_extended.py with 37 tests covering log rotation,
  date range queries, custom filters, and compliance reporting
- Total test coverage now 90.08% (1,840 tests passing)

Key coverage improvements:
- secure_memdocs.py: 96.12%
- audit_logger.py: 95.29%
- secrets_detector.py: 94.98%
- pii_scrubber.py: 97.84%

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: 2 people

empathy_llm_toolkit/security/secure_memdocs.py

@@ -41,7 +41,7 @@
 
 import structlog
 
-from .audit_logger import AuditLogger
+from .audit_logger import AuditEvent, AuditLogger
 from .pii_scrubber import PIIScrubber
 from .secrets_detector import SecretsDetector
 
@@ -1133,7 +1133,7 @@ def delete_pattern(self, pattern_id: str, user_id: str, session_id: str = "") ->
         if deleted:
             # Log deletion
             self.audit_logger._write_event(
-                self.audit_logger.AuditEvent(
+                AuditEvent(
                     event_type="delete_pattern",
                     user_id=user_id,
                     session_id=session_id,