fix(stepfun): update provider endpoints and pricing

[cantalupo555]

Summary

This PR updates the StepFun provider configuration to reflect their platform migration from stepfun.com to stepfun.ai. The API endpoint, documentation URL, and pricing for the step-3.5-flash model are updated to match the current official documentation.

Changes

• Provider configuration: Updated API endpoint from https://api.stepfun.com/v1 to https://api.stepfun.ai/v1
• Documentation URL: Updated docs link from Chinese to English documentation at https://platform.stepfun.ai/docs/en/guides/basic-concepts
• Pricing updates: Adjusted step-3.5-flash model pricing (input: $0.096→$0.10, output: $0.288→$0.30, cache_read: $0.019→$0.02 per 1M tokens)

Breaking Changes

• API endpoint changed from api.stepfun.com to api.stepfun.ai. This is a provider-mandated migration — the old domain may stop working. Users should update their configurations.

Testing

• All configurations validated with bun validate

[rekram1-node]

i know nothing about the provider, are u sure thats the correct urls? I need some evidence cause this could be an attack vector

[cantalupo555]

@rekram1-node
Yes, I took great care to review the updated documentation.
https://platform.stepfun.ai/docs/en/api-reference/chat/chat-completion-create

curl https://api.stepfun.ai/v1/models \
  -H "Authorization: Bearer 2yzIOTelUt[...]Pwz"

{
  "data": [
    {
      "id": "step-tts-2",
      "object": "model",
      "created": 1766113244,
      "owned_by": "stepai"
    },
    {
      "id": "step-3.5-flash",
      "object": "model",
      "created": 1769580058,
      "owned_by": "stepai"
    },
    {
      "id": "step-3.5-flash-paid",
      "object": "model",
      "created": 1770681764,
      "owned_by": "stepai"
    },
    {
      "id": "step-3.5-flash-2603",
      "object": "model",
      "created": 1774709640,
      "owned_by": "stepai"
    }
  ],
  "object": "list"
}

[cantalupo555]

@rekram1-node I made a discovery: the .com domain is the Chinese version of the API.
The .ai domain is the Western version.

---

https://platform.stepfun.com/

https://platform.stepfun.ai/

[cantalupo555]

Sorry, I'm going to change this PR from fix to chore.

The endpoint api.stepfun.com is correct; it's based in China.
I'll leave the endpoint api.stepfun.ai (Western server) for another PR.

https://platform.stepfun.com/docs/zh/api-reference/chat/chat-completion-create

https://platform.stepfun.ai/docs/en/api-reference/chat/chat-completion-create