feat(03-01): add 8 Tier 4 providers (Baichuan, StepFun, SenseTime, iFlytek, Tencent, SiliconFlow, 360AI, Kuaishou)

- SiliconFlow uses documented sk- prefix
- Other 7 keyword-only (no documented key format, avoids false positives)
- Completes PROV-04: 16 Tier 4 Chinese/regional providers

providers/baichuan.yaml

@@ -0,0 +1,18 @@
+format_version: 1
+name: baichuan
+display_name: Baichuan AI
+tier: 4
+last_verified: "2026-04-05"
+keywords:
+  - "baichuan"
+  - "BAICHUAN_API_KEY"
+  - "api.baichuan-ai.com"
+  - "baichuan2"
+  - "baichuan-turbo"
+verify:
+  method: GET
+  url: https://api.baichuan-ai.com/v1/chat/completions
+  headers:
+    Authorization: "Bearer {KEY}"
+  valid_status: [200, 400]
+  invalid_status: [401, 403]