P0-1: Wire up guardrail config loading from etcd / YAML

Problem

crates/aisix-guardrails/ ships keyword/regex/length/chain rules and the Guardrail trait is wired into the proxy hot path (crates/aisix-proxy/src/state.rs:40, called at chat.rs:184 input + chat.rs:393 output). But:

• aisix-core/src/config.rs has no guardrails: section.
• crates/aisix-server/src/main.rs never calls with_guardrails.
• Default state ships GuardrailChain::empty() (state.rs:62,85,111).

Result: an operator literally cannot turn on guardrails outside of unit-test code paths — it's scaffold-only.

Scope

This is the 0→1 fix to make every other guardrail issue (Bedrock, Lakera, Presidio, OpenAI Moderation) actually plug-in-able.

DP (moonming/ai-gateway)

• Add guardrails: config section to Config (aisix-core/src/config.rs).
• Add etcd resource kind guardrails parallel to models / apikeys in the snapshot loader (crates/aisix-etcd/src/loader.rs).
• Construct GuardrailChain at startup from config + etcd, wire via with_guardrails.

CP (api7/api7ee-3-control-plane)

• New CRUD endpoints /api/ai_gateway_groups/:gid/guardrails[/:id] (env-scoped, per api7ee-3-control-plane#2625).
• Persist + push to kine under /aisix/<group>/guardrails/<id>.

Dashboard UI (api7/AISIX-Cloud)

• Add Guardrails page under Environment scope.
• CRUD form: type (keyword/regex/length/chain) + config JSON editor + applies-to model picker.

e2e

• Go: create guardrail via SaaS API → verify DP enforces (block keyword in chat completion → expect rejection).
• Playwright: dashboard create/edit/delete guardrail flow.

Estimate

2d (DP-only); +2d CP, +1.5d UI, +0.5d e2e

Start by reading aisix-core/src/config.rs, crates/aisix-etcd/src/loader.rs, crates/aisix-server/src/main.rs, and the GuardrailChain defaults and call sites in crates/aisix-proxy. Trace how models and API keys are loaded, then verify the stated DP, control-plane, dashboard, and e2e flows are covered. Done means configured guardrails reach the proxy and the CRUD and verification flows work across the listed components.