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

pkg/providers/definitions/stepfun.yaml

@@ -0,0 +1,18 @@
+format_version: 1
+name: stepfun
+display_name: StepFun (阶跃星辰)
+tier: 4
+last_verified: "2026-04-05"
+keywords:
+  - "stepfun"
+  - "STEP_API_KEY"
+  - "STEPFUN_API_KEY"
+  - "api.stepfun.com"
+  - "step-1v"
+verify:
+  method: GET
+  url: https://api.stepfun.com/v1/models
+  headers:
+    Authorization: "Bearer {KEY}"
+  valid_status: [200]
+  invalid_status: [401, 403]