#10857: Add model provider StepFun

## Summary Refactor StepFun onboarding to handle multiple endpoints via auth choices (GLM-style), instead of introducing StepFun-specific onboarding parameters. This PR adds StepFun as a provider in onboarding/CLI/docs and supports model `step-3.5-flash`. ## What changed - Added StepFun auth choices: - `stepfun-api-key` (default/global endpoint) - `stepfun-cn` (China endpoint) - Removed StepFun-specific onboarding flag/type plumbing (`--stepfun-endpoint`). - Resolved endpoint selection through auth-choice mapping in both interactive and non-interactive onboarding paths. - Preserved existing `models.providers.stepfun.baseUrl` on reruns when using `stepfun-api-key`. - Updated docs/CLI references for the auth-choice-based endpoint handling. ## User-facing behavior - Default flow stays simple: select StepFun and provide API key. - CN endpoint is available via the dedicated auth choice when needed. - Existing StepFun configs are not clobbered by rerunning onboarding with the default StepFun auth choice. ## Testing performed - `pnpm test src/commands/auth-choice.stepfun.test.ts src/commands/onboard-non-interactive.stepfun.test.ts` - `pnpm exec vitest run --config vitest.e2e.config.ts src/commands/onboard-non-interactive.provider-auth.e2e.test.ts` - `pnpm exec oxfmt --check src/cli/program/register.onboard.ts src/commands/auth-choice-options.e2e.test.ts src/commands/auth-choice-options.ts src/commands/auth-choice.apply.api-providers.ts src/commands/auth-choice.apply.ts src/commands/auth-choice.preferred-provider.ts src/commands/auth-choice.stepfun.test.ts src/commands/onboard-non-interactive.stepfun.test.ts src/commands/onboard-non-interactive/local/auth-choice.ts src/commands/onboard-types.ts src/wizard/onboarding.ts` ## Screenshot I will refresh the onboarding screenshot in a follow-up update.