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/main.py

@@ -1566,6 +1566,8 @@ def select_provider_and_model(args=None):
         _model_flow_anthropic(config, current_model)
     elif selected_provider == "kimi-coding":
         _model_flow_kimi(config, current_model)
+    elif selected_provider == "stepfun":
+        _model_flow_stepfun(config, current_model)
     elif selected_provider == "bedrock":
         _model_flow_bedrock(config, current_model)
     elif selected_provider in (
@@ -3462,6 +3464,140 @@ def _model_flow_kimi(config, current_model=""):
         print("No change.")
 
 
+def _infer_stepfun_region(base_url: str) -> str:
+    """Infer the current StepFun region from the configured endpoint."""
+    normalized = (base_url or "").strip().lower()
+    if "api.stepfun.com" in normalized:
+        return "china"
+    return "international"
+
+
+def _stepfun_base_url_for_region(region: str) -> str:
+    from hermes_cli.auth import (
+        STEPFUN_STEP_PLAN_CN_BASE_URL,
+        STEPFUN_STEP_PLAN_INTL_BASE_URL,
+    )
+
+    return (
+        STEPFUN_STEP_PLAN_CN_BASE_URL
+        if region == "china"
+        else STEPFUN_STEP_PLAN_INTL_BASE_URL
+    )
+
+
+def _model_flow_stepfun(config, current_model=""):
+    """StepFun Step Plan flow with region-specific endpoints."""
+    from hermes_cli.auth import (
+        PROVIDER_REGISTRY,
+        _prompt_model_selection,
+        _save_model_choice,
+        deactivate_provider,
+    )
+    from hermes_cli.config import get_env_value, save_env_value, load_config, save_config
+    from hermes_cli.models import fetch_api_models
+
+    provider_id = "stepfun"
+    pconfig = PROVIDER_REGISTRY[provider_id]
+    key_env = pconfig.api_key_env_vars[0] if pconfig.api_key_env_vars else ""
+    base_url_env = pconfig.base_url_env_var or ""
+
+    existing_key = ""
+    for ev in pconfig.api_key_env_vars:
+        existing_key = get_env_value(ev) or os.getenv(ev, "")
+        if existing_key:
+            break
+
+    if not existing_key:
+        print(f"No {pconfig.name} API key configured.")
+        if key_env:
+            try:
+                import getpass
+                new_key = getpass.getpass(f"{key_env} (or Enter to cancel): ").strip()
+            except (KeyboardInterrupt, EOFError):
+                print()
+                return
+            if not new_key:
+                print("Cancelled.")
+                return
+            save_env_value(key_env, new_key)
+            existing_key = new_key
+            print("API key saved.")
+            print()
+    else:
+        print(f"  {pconfig.name} API key: {existing_key[:8]}... ✓")
+        print()
+
+    current_base = ""
+    if base_url_env:
+        current_base = get_env_value(base_url_env) or os.getenv(base_url_env, "")
+    if not current_base:
+        model_cfg = config.get("model")
+        if isinstance(model_cfg, dict):
+            current_base = str(model_cfg.get("base_url") or "").strip()
+    current_region = _infer_stepfun_region(current_base or pconfig.inference_base_url)
+
+    region_choices = [
+        ("international", f"International ({_stepfun_base_url_for_region('international')})"),
+        ("china", f"China ({_stepfun_base_url_for_region('china')})"),
+    ]
+    ordered_regions = []
+    for region_key, label in region_choices:
+        if region_key == current_region:
+            ordered_regions.insert(0, (region_key, f"{label}  ← currently active"))
+        else:
+            ordered_regions.append((region_key, label))
+    ordered_regions.append(("cancel", "Cancel"))
+
+    region_idx = _prompt_provider_choice([label for _, label in ordered_regions])
+    if region_idx is None or ordered_regions[region_idx][0] == "cancel":
+        print("No change.")
+        return
+
+    selected_region = ordered_regions[region_idx][0]
+    effective_base = _stepfun_base_url_for_region(selected_region)
+    if base_url_env:
+        save_env_value(base_url_env, effective_base)
+
+    live_models = fetch_api_models(existing_key, effective_base)
+    if live_models:
+        model_list = live_models
+        print(f"  Found {len(model_list)} model(s) from {pconfig.name} API")
+    else:
+        model_list = _PROVIDER_MODELS.get(provider_id, [])
+        if model_list:
+            print(
+                f"  Could not auto-detect models from {pconfig.name} API — "
+                "showing Step Plan fallback catalog."
+            )
+
+    if model_list:
+        selected = _prompt_model_selection(model_list, current_model=current_model)
+    else:
+        try:
+            selected = input("Model name: ").strip()
+        except (KeyboardInterrupt, EOFError):
+            selected = None
+
+    if selected:
+        _save_model_choice(selected)
+
+        cfg = load_config()
+        model = cfg.get("model")
+        if not isinstance(model, dict):
+            model = {"default": model} if model else {}
+            cfg["model"] = model
+        model["provider"] = provider_id
+        model["base_url"] = effective_base
+        model.pop("api_mode", None)
+        save_config(cfg)
+        deactivate_provider()
+
+        config["model"] = dict(model)
+        print(f"Default model set to: {selected} (via {pconfig.name})")
+    else:
+        print("No change.")
+
+
 def _model_flow_bedrock_api_key(config, region, current_model=""):
     """Bedrock API Key mode — uses the OpenAI-compatible bedrock-mantle endpoint.
 
@@ -6530,6 +6666,7 @@ For more help on a command:
             "zai",
             "kimi-coding",
             "kimi-coding-cn",
+            "stepfun",
             "minimax",
             "minimax-cn",
             "kilocode",