feat: add Step Plan provider support (salvage #6005)

Adds a first-class 'stepfun' API-key provider surfaced as Step Plan:

- Support Step Plan setup for both International and China regions
- Discover Step Plan models live from /step_plan/v1/models, with a
  small coding-focused fallback catalog when discovery is unavailable
- Thread StepFun through provider metadata, setup persistence, status
  and doctor output, auxiliary routing, and model normalization
- Add tests for provider resolution, model validation, metadata
  mapping, and StepFun region/model persistence

Based on #6005 by @hengm3467.

Co-authored-by: hengm3467 <100685635+hengm3467@users.noreply.github.com>

tests/agent/test_models_dev.py

@@ -82,6 +82,7 @@ class TestProviderMapping:
     def test_known_providers_mapped(self):
         assert PROVIDER_TO_MODELS_DEV["anthropic"] == "anthropic"
         assert PROVIDER_TO_MODELS_DEV["copilot"] == "github-copilot"
+        assert PROVIDER_TO_MODELS_DEV["stepfun"] == "stepfun"
         assert PROVIDER_TO_MODELS_DEV["kilocode"] == "kilo"
         assert PROVIDER_TO_MODELS_DEV["ai-gateway"] == "vercel"