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/status.py

@@ -122,6 +122,7 @@ def show_status(args):
         "OpenAI": "OPENAI_API_KEY",
         "Z.AI/GLM": "GLM_API_KEY",
         "Kimi": "KIMI_API_KEY",
+        "StepFun Step Plan": "STEPFUN_API_KEY",
         "MiniMax": "MINIMAX_API_KEY",
         "MiniMax-CN": "MINIMAX_CN_API_KEY",
         "Firecrawl": "FIRECRAWL_API_KEY",
@@ -252,6 +253,7 @@ def show_status(args):
     apikey_providers = {
         "Z.AI / GLM":       ("GLM_API_KEY", "ZAI_API_KEY", "Z_AI_API_KEY"),
         "Kimi / Moonshot":  ("KIMI_API_KEY",),
+        "StepFun Step Plan": ("STEPFUN_API_KEY",),
         "MiniMax":          ("MINIMAX_API_KEY",),
         "MiniMax (China)":  ("MINIMAX_CN_API_KEY",),
     }