fix(security): deduplicate deny reasons, configurable cleanup, injection threshold audit, redact auth header

[imran-siddique]

Summary

• LOW: Cedar deny reasons list not deduplicated — duplicate deny reasons inflate audit log (POLICY-008) #186 (POLICY-008): Deduplicate deny_reasons with dict.fromkeys() before computing final verdict in InspectionPipeline, so the audit record reflects distinct policy firings rather than duplicated entries from overlapping code paths.
• LOW: Session cleanup interval not configurable — fixed 60s period ignores load pattern (AUTH-004) #187 (AUTH-004): Expose CMCP_SESSION_CLEANUP_INTERVAL_SECONDS env var (int, default 60) as a class attribute on SessionManager and an instance attribute on MCPServer, making the cleanup timer configurable without code changes.
• LOW: AGT MCPResponseScanner score threshold not logged — threshold changes are silent (INJECT-007) #191 (INJECT-007): Propagate injection_threshold from AGT MCPResponseScanner through proxy.py into the injection_detail audit dict, so downstream consumers can replay the allow/deny decision under config changes.
• LOW: OPAQUE_API_KEY logged at DEBUG level if httpx request fails — key appears in debug logs (HW-008) #194 (HW-008): Add _redact_auth_headers() to cmcp_verify/opaque.py and use it in all debug log calls; add OPAQUE_API_KEY env var support that injects a Bearer token into Opaque attestation requests without ever logging the raw value.

Test plan

• tests/unit/test_low_batch_186_187_191_194.py — 15 new unit tests, all passing locally
  • test_deny_reasons_deduplication / test_deny_reasons_single_entry / test_deny_reasons_empty (POLICY-008)
  • test_session_manager_cleanup_interval_default / test_session_manager_cleanup_interval_from_env / test_mcp_server_cleanup_interval_default / test_mcp_server_cleanup_interval_from_env (AUTH-004)
  • test_injection_threshold_present_for_deny / test_injection_threshold_strict / test_injection_threshold_permissive / test_agt_mcp_scanner_deny_includes_threshold (INJECT-007)
  • test_redact_auth_headers_redacts_authorization / test_redact_auth_headers_case_insensitive / test_redact_auth_headers_no_auth_unchanged / test_opaque_api_key_not_logged_on_failure / test_opaque_verify_sends_api_key_as_bearer (HW-008)
• Run pytest tests/unit/ — pre-existing failures in test_benchmarks, test_tee_dev_mode_freeze, test_tee_providers are unrelated to this PR

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