[Good first issue]: Add a retrieval-poisoning Failure Zoo specimen

[thangldw]

User outcome

A contributor can add one inspectable retrieval-poisoning regression to the RAG Failure Zoo and see RAGOps block it for a named, deterministic reason.

Scope

• Add one synthetic case or response override under scenarios/japanese_troubleshooting/.
• Preserve a passing baseline and add one intentionally poisoned candidate.
• Add the specimen to docs/evaluation/benchmark.md.
• Add expected and negative tests.

Acceptance criteria

• The baseline passes.
• The poisoned candidate exits 2 with a named finding or gate.
• The fixture contains no real secrets, customer data, or live URLs.
• ruff check . and PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest -q pass.

Non-goals

Do not claim that one synthetic specimen proves production security or broad prompt-injection resistance. Do not change a public schema or default gate.