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/models.py

@@ -210,6 +210,10 @@ _PROVIDER_MODELS: dict[str, list[str]] = {
         "kimi-k2-turbo-preview",
         "kimi-k2-0905-preview",
     ],
+    "stepfun": [
+        "step-3.5-flash",
+        "step-3.5-flash-2603",
+    ],
     "moonshot": [
         "kimi-k2.6",
         "kimi-k2.5",
@@ -699,6 +703,7 @@ CANONICAL_PROVIDERS: list[ProviderEntry] = [
     ProviderEntry("zai",            "Z.AI / GLM",               "Z.AI / GLM (Zhipu AI direct API)"),
     ProviderEntry("kimi-coding",    "Kimi / Kimi Coding Plan",  "Kimi Coding Plan (api.kimi.com) & Moonshot API"),
     ProviderEntry("kimi-coding-cn", "Kimi / Moonshot (China)",  "Kimi / Moonshot China (Moonshot CN direct API)"),
+    ProviderEntry("stepfun",        "StepFun Step Plan",       "StepFun Step Plan (agent/coding models via Step Plan API)"),
     ProviderEntry("minimax",        "MiniMax",                  "MiniMax (global direct API)"),
     ProviderEntry("minimax-cn",     "MiniMax (China)",          "MiniMax China (domestic direct API)"),
     ProviderEntry("alibaba",        "Alibaba Cloud (DashScope)","Alibaba Cloud / DashScope Coding (Qwen + multi-provider)"),
@@ -733,6 +738,8 @@ _PROVIDER_ALIASES = {
     "moonshot": "kimi-coding",
     "kimi-cn": "kimi-coding-cn",
     "moonshot-cn": "kimi-coding-cn",
+    "step": "stepfun",
+    "stepfun-coding-plan": "stepfun",
     "arcee-ai": "arcee",
     "arceeai": "arcee",
     "minimax-china": "minimax-cn",
@@ -1613,6 +1620,19 @@ def provider_model_ids(provider: Optional[str], *, force_refresh: bool = False)
                     return live
         except Exception:
             pass
+    if normalized == "stepfun":
+        try:
+            from hermes_cli.auth import resolve_api_key_provider_credentials
+
+            creds = resolve_api_key_provider_credentials("stepfun")
+            api_key = str(creds.get("api_key") or "").strip()
+            base_url = str(creds.get("base_url") or "").strip()
+            if api_key and base_url:
+                live = fetch_api_models(api_key, base_url)
+                if live:
+                    return live
+        except Exception:
+            pass
     if normalized == "anthropic":
         live = _fetch_anthropic_models()
         if live: