fix(gateway): harden provider redaction boundaries

[dmoliveira]

Summary

• Reject detector-matching, blank, or oversized redaction tokens.
• Charge positive replacement expansion and fail closed on residual detector matches.
• Keep immutable provider traversal sticky and reject malformed provider roots.
• Hash bounded audit session IDs without persisting raw values.

Validation Evidence

• npm test (967 passing)
• npm run lint
• make validate
• pre-commit run --all-files
• git diff --check
• Final static security review: no blocker findings.