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/providers.py

@@ -94,6 +94,12 @@ HERMES_OVERLAYS: Dict[str, HermesOverlay] = {
         transport="openai_chat",
         base_url_env_var="KIMI_BASE_URL",
     ),
+    "stepfun": HermesOverlay(
+        transport="openai_chat",
+        extra_env_vars=("STEPFUN_API_KEY",),
+        base_url_override="https://api.stepfun.ai/step_plan/v1",
+        base_url_env_var="STEPFUN_BASE_URL",
+    ),
     "minimax": HermesOverlay(
         transport="anthropic_messages",
         base_url_env_var="MINIMAX_BASE_URL",
@@ -210,6 +216,10 @@ ALIASES: Dict[str, str] = {
     "kimi-coding-cn": "kimi-for-coding",
     "moonshot": "kimi-for-coding",
 
+    # stepfun
+    "step": "stepfun",
+    "stepfun-coding-plan": "stepfun",
+
     # minimax-cn
     "minimax-china": "minimax-cn",
     "minimax_cn": "minimax-cn",
@@ -294,6 +304,7 @@ _LABEL_OVERRIDES: Dict[str, str] = {
     "nous": "Nous Portal",
     "openai-codex": "OpenAI Codex",
     "copilot-acp": "GitHub Copilot ACP",
+    "stepfun": "StepFun Step Plan",
     "xiaomi": "Xiaomi MiMo",
     "local": "Local endpoint",
     "bedrock": "AWS Bedrock",