ai_guardrails: NeMo mask / redact action

[liavweiss]

Description

Implement the redaction flow: when NeMo returns "modified" status,
extract the masked content and replace the original request body.
Link for the Proposal: docs/proposals/00138_external_guardrails.md

• When NeMo returns "modified" status, the redacted content is available in the NeMo response at: guardrails_data.log.activated_rails.executed_actions.return_value (need to loop into all executed_actions)
• NemoProvider extracts this redacted text and returns it as GuardResult::Redact { modified_text, reason }
• In filter.rs: replace the original body bytes with the redacted content, update Content-Length header
• For request-side: replace the last user message content in the request body with the redacted version

NeMo deployment for testing:

• TrustyAI Helm chart
• Debug image for E2E testing: quay.io/rgeada/nemo-debug:latest (quay.io)

Acceptance:

• Request containing PII/sensitive content is forwarded with masked body when NeMo returns "modified"
• Content-Length header is updated to match the new body size
• FilterResultSet status is written as "redacted"

Dependencies

• Depends on "ai_guardrails: NeMo provider and request-side integration"

Motivation

Pass and block are binary outcomes, but many guardrail policies need a
third option: mask sensitive content and forward the sanitized request.
NeMo supports this via its "modified" status, and the filter needs to
extract the redacted text and rewrite the body in-flight.

Alternatives Considered

Rejecting the entire request when sensitive content is detected instead
of redacting. This is already supported via the Block result; redaction
adds a less disruptive option for operators who want to sanitize rather
than reject.

Area

Filter Pipeline

Parent Epic (optional)

#76

[rkaplan-hub]

/assign