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/auth.py

@@ -72,6 +72,8 @@ DEFAULT_QWEN_BASE_URL = "https://portal.qwen.ai/v1"
 DEFAULT_GITHUB_MODELS_BASE_URL = "https://api.githubcopilot.com"
 DEFAULT_COPILOT_ACP_BASE_URL = "acp://copilot"
 DEFAULT_OLLAMA_CLOUD_BASE_URL = "https://ollama.com/v1"
+STEPFUN_STEP_PLAN_INTL_BASE_URL = "https://api.stepfun.ai/step_plan/v1"
+STEPFUN_STEP_PLAN_CN_BASE_URL = "https://api.stepfun.com/step_plan/v1"
 CODEX_OAUTH_CLIENT_ID = "app_EMoamEEZ73f0CkXaXp7hrann"
 CODEX_OAUTH_TOKEN_URL = "https://auth.openai.com/oauth/token"
 CODEX_ACCESS_TOKEN_REFRESH_SKEW_SECONDS = 120
@@ -182,6 +184,14 @@ PROVIDER_REGISTRY: Dict[str, ProviderConfig] = {
         inference_base_url="https://api.moonshot.cn/v1",
         api_key_env_vars=("KIMI_CN_API_KEY",),
     ),
+    "stepfun": ProviderConfig(
+        id="stepfun",
+        name="StepFun Step Plan",
+        auth_type="api_key",
+        inference_base_url=STEPFUN_STEP_PLAN_INTL_BASE_URL,
+        api_key_env_vars=("STEPFUN_API_KEY",),
+        base_url_env_var="STEPFUN_BASE_URL",
+    ),
     "arcee": ProviderConfig(
         id="arcee",
         name="Arcee AI",
@@ -992,6 +1002,7 @@ def resolve_provider(
         "x-ai": "xai", "x.ai": "xai", "grok": "xai",
         "kimi": "kimi-coding", "kimi-for-coding": "kimi-coding", "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", "minimax_cn": "minimax-cn",
         "claude": "anthropic", "claude-code": "anthropic",