test(guardrails): pin monitor mode against provider failures

[jarvis9443]

What

Regression coverage for the enforcement_mode: "monitor" contract under provider failure, prompted by AISIX-Cloud#1010 (monitor-mode Aliyun guardrail reported intermittently returning 422).

Verified against current main (identical guardrail code to v0.3.1): the behavior is already correct — monitor mode downgrades every block, including a fail-closed provider outage. What was missing is any test pinning that interaction; existing monitor tests only covered content flags on in-process guardrails (keyword/pii), never a failing remote provider.

Tests added

e2e (guardrail-monitor-provider-failure-e2e.test.ts, real aisix + etcd + mock green-cip endpoint):

1. block + fail_open: false + hard-down moderation endpoint → 422 content_filter (the only guardrail-path mechanism that can produce the symptom reported in the issue; doubles as the propagation gate)
2. same guardrail flipped to monitor → traffic flows again, upstream reached, provider still consulted
3. monitor + a flag-everything provider → observed, not blocked

unit (build.rs, composed through build_one so the decorator ordering itself is pinned):

• monitor downgrades a fail-closed provider-failure Block
• mandatory: true keeps provider unavailability fatal in monitor mode (the documented outermost-decorator exception)

Notes

• Test-only; no behavior change.
• Ref api7/AISIX-Cloud#1010 (intentionally not Fixes — root cause in the reported environment is still being confirmed on the issue; these tests pin the current-code contract).

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

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Run ID: 388927de-a8a0-4a03-aea2-0d08aafdf400

📥 Commits

Reviewing files that changed from the base of the PR and between 4ddd6ad and ff35817.

📒 Files selected for processing (2)

• crates/aisix-guardrails/src/build.rs
• tests/e2e/src/cases/guardrail-monitor-provider-failure-e2e.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/guardrail-monitor-provider-failure

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