feat(catalog): add StepFun providers

[jimpablo]

Motivation

Add StepFun Step-series reasoning models to the provider catalog, following the domestic/international split used by minimax, moonshot, and xiaomi.

What's added

Four openai-compatible providers, all serving step-3.7-flash, step-3.5-flash, and step-3.5-flash-2603:

Provider	Base URL	Region / plan
stepfun	https://api.stepfun.ai/v1	International, standard API
stepfun-cn	https://api.stepfun.com/v1	China, standard API
stepfun-step-plan	https://api.stepfun.ai/step_plan/v1	International, Step Plan
stepfun-step-plan-cn	https://api.stepfun.com/step_plan/v1	China, Step Plan

• step-3.7-flash is multimodal (text + image); the step-3.5-flash variants are text-only.
• Reasoning via reasoning_effort (low / medium / high only — StepFun exposes no off/minimal).
• 256k context window for all models.

Pricing

USD per 1M tokens, same for all four providers:

Model	Input	Cached input	Output
step-3.7-flash	0.20	0.04	1.15
step-3.5-flash	0.10	0.02	0.30
step-3.5-flash-2603	0.10	0.02	0.30

Checks

• uv run pytest — 740 passed
• uv run ruff check . / ruff format --check . — clean
• uv run mypy — clean

Catalog-only change; no Python or architectural changes.