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>

tests/hermes_cli/test_model_validation.py

@@ -63,6 +63,11 @@ class TestParseModelInput:
         assert provider == "zai"
         assert model == "glm-5"
 
+    def test_stepfun_alias_resolved(self):
+        provider, model = parse_model_input("step:step-3.5-flash", "openrouter")
+        assert provider == "stepfun"
+        assert model == "step-3.5-flash"
+
     def test_no_slash_no_colon_keeps_provider(self):
         provider, model = parse_model_input("gpt-5.4", "openrouter")
         assert provider == "openrouter"
@@ -154,6 +159,7 @@ class TestNormalizeProvider:
         assert normalize_provider("glm") == "zai"
         assert normalize_provider("kimi") == "kimi-coding"
         assert normalize_provider("moonshot") == "kimi-coding"
+        assert normalize_provider("step") == "stepfun"
         assert normalize_provider("github-copilot") == "copilot"
 
     def test_case_insensitive(self):
@@ -164,6 +170,7 @@ class TestProviderLabel:
     def test_known_labels_and_auto(self):
         assert provider_label("anthropic") == "Anthropic"
         assert provider_label("kimi") == "Kimi / Kimi Coding Plan"
+        assert provider_label("stepfun") == "StepFun Step Plan"
         assert provider_label("copilot") == "GitHub Copilot"
         assert provider_label("copilot-acp") == "GitHub Copilot ACP"
         assert provider_label("auto") == "Auto"
@@ -193,6 +200,16 @@ class TestProviderModelIds:
     def test_zai_returns_glm_models(self):
         assert "glm-5" in provider_model_ids("zai")
 
+    def test_stepfun_prefers_live_catalog(self):
+        with patch(
+            "hermes_cli.auth.resolve_api_key_provider_credentials",
+            return_value={"api_key": "***", "base_url": "https://api.stepfun.com/step_plan/v1"},
+        ), patch(
+            "hermes_cli.models.fetch_api_models",
+            return_value=["step-3.5-flash", "step-3-agent-lite"],
+        ):
+            assert provider_model_ids("stepfun") == ["step-3.5-flash", "step-3-agent-lite"]
+
     def test_copilot_prefers_live_catalog(self):
         with patch("hermes_cli.auth.resolve_api_key_provider_credentials", return_value={"api_key": "gh-token"}), \
              patch("hermes_cli.models._fetch_github_models", return_value=["gpt-5.4", "claude-sonnet-4.6"]):