feat(models): add StepFun reasoning model adapter (#3461)

Add PatchedChatStepFun adapter for StepFun reasoning models (step-3.7-flash,
step-3.5-flash). Captures reasoning from both streaming and non-streaming
responses and replays it on historical assistant messages for multi-turn
tool-call conversations.

- New: PatchedChatStepFun adapter with streaming/non-streaming reasoning capture
- Support both reasoning and reasoning_content field names
- 17 unit tests covering all response paths
- Updated: config.example.yaml with StepFun configuration example

config.example.yaml

@@ -274,6 +274,32 @@ models:
   #       thinking:
   #         type: disabled
 
+  # Example: StepFun (阶跃星辰) reasoning models
+  # StepFun provides OpenAI-compatible API with reasoning models.
+  # With reasoning_format: deepseek-style, the API returns reasoning_content
+  # (same field as DeepSeek), which must be replayed on historical assistant
+  # messages in multi-turn tool-call conversations.
+  # Use PatchedChatStepFun instead of plain ChatOpenAI.
+  # Docs: https://platform.stepfun.com/docs/api-reference/chat-completions
+  # - name: step-3.7-flash
+  #   display_name: Step 3.7 Flash
+  #   use: deerflow.models.patched_stepfun:PatchedChatStepFun
+  #   model: step-3.7-flash
+  #   api_key: $STEPFUN_API_KEY
+  #   base_url: https://api.stepfun.com/v1
+  #   request_timeout: 600.0
+  #   max_retries: 2
+  #   max_tokens: 4096
+  #   supports_thinking: true
+  #   supports_reasoning_effort: true
+  #   supports_vision: true
+  #   when_thinking_enabled:
+  #     extra_body:
+  #       reasoning_format: deepseek-style
+  #   when_thinking_disabled:
+  #     extra_body:
+  #       reasoning_format: deepseek-style
+
   # Example: MiniMax (OpenAI-compatible) - International Edition
   # MiniMax provides high-performance models with 512K context window and 128K max output
   # Docs: https://platform.minimax.io/docs/api-reference/text-openai-api