feat(pii): restore request-scoped pseudonyms

[localai-org-maint-bot]

Description

Closes #9535.

Adds opt-in pii.reverse_in_response handling to the existing detector-driven PII middleware. Masked request values become unique, deterministic pseudonyms within the request (EMAIL_001, EMAIL_002) and are restored if the backend returns them. Restoration works for normal JSON and SSE tokens split across response writes; substitution maps remain request-local and are never persisted.

The model config metadata and middleware documentation expose the new toggle. Existing irreversible [REDACTED:...] behavior remains the default.

Notes for Reviewers

• go test ./core/services/routing/pii ./core/config/meta
• go vet ./core/services/routing/pii ./core/config/meta
• The full core/config suite reaches 433/434 specs; its pre-existing remote-config validation spec fails because raw.githubusercontent.com returns HTTP 403 in this runner.

Signed commits

• Yes, I signed my commits.
• Documentation updated (docs/content/) for user-facing changes, or not applicable

[richiejp]

I think the name could be better. Something like "reversible redactions" or there may be a common term in usage for this already.

In the docs I think we should note that doing this makes it easier for a third party to reverse the redactions as well.

[richiejp]

markers like EMAIL_001 are very likely to collide with other text in the model output like [REDACTED:EMAIL_001] or similar won't, although the model may choose to remove the REDACTED part. Wrapping the ID with a prefix and postfix should probably be config options.