feat(providers): add StepFun services

[PeronGH]

Summary

• add StepFun direct API services for the global and China endpoints
• add Step Plan subscription endpoints with localized labels and hints
• verify account creation submits the matching OpenAI-compatible endpoint and API key

Closes CODE-571

Verification

• pnpm test packages/client/workbench/src/settings/providers/__tests__/catalog.test.ts packages/client/workbench/src/settings/providers/__tests__/add-flow.test.tsx (12 tests passed before the redundant catalog test was removed)
• pre-commit formatting, lint, and typecheck hooks passed

Checklist

• pnpm check:ci and pnpm test both pass (targeted tests and commit hooks passed)
• I ran the affected surface and observed the change working
• If a wire message changed: WIRE_PROTOCOL_VERSION is bumped (no wire change)
• New code and assets are my own work, or their origin and license compatibility are noted above
• Docs and comments are updated where behavior changed

[linear-code]

CODE-571

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[PeronGH]

Closed due to conflicts with #424