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>

hermes_cli/model_switch.py

@@ -143,7 +143,7 @@ MODEL_ALIASES: dict[str, ModelIdentity] = {
     # Z.AI / GLM
     "glm":       ModelIdentity("z-ai", "glm"),
 
-    # StepFun
+    # Step Plan (StepFun)
     "step":      ModelIdentity("stepfun", "step"),
 
     # Xiaomi