feat: add reasoning_effort configuration support for Doubao/GPT-5 models (#947)

* feat: Add reasoning effort configuration support

* Add `reasoning_effort` parameter to model config and agent initialization
* Support reasoning effort levels (minimal/low/medium/high) for Doubao/GPT-5 models
* Add UI controls in input box for reasoning effort selection
* Update doubao-seed-1.8 example config with reasoning effort support

Fixes & Cleanup:
* Ensure UTF-8 encoding for file operations
* Remove unused imports

* fix: set reasoning_effort to None for unsupported models

* fix: unit test error

* Update frontend/src/components/workspace/input-box.tsx

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

config.example.yaml

@@ -65,18 +65,19 @@ models:
   #         type: enabled
 
   # Example: Volcengine (Doubao) model
-  # - name: doubao-seed-1.8
-  #   display_name: Doubao 1.8 (Thinking)
-  #   use: langchain_deepseek:ChatDeepSeek
-  #   model: ep-m-20260106111913-xxxxx
-  #   api_base: https://ark.cn-beijing.volces.com/api/v3
-  #   api_key: $VOLCENGINE_API_KEY
-  #   supports_thinking: true
-  #   supports_vision: false  # Check your specific model's capabilities
-  #   when_thinking_enabled:
-  #     extra_body:
-  #       thinking:
-  #         type: enabled
+  - name: doubao-seed-1.8
+    display_name: Doubao-Seed-1.8
+    use: src.models.patched_deepseek:PatchedChatDeepSeek
+    model: doubao-seed-1-8-251228
+    api_base: https://ark.cn-beijing.volces.com/api/v3
+    api_key: $VOLCENGINE_API_KEY
+    supports_thinking: true
+    supports_vision: true
+    supports_reasoning_effort: true
+    when_thinking_enabled:
+      extra_body:
+        thinking:
+          type: enabled
 
   # Example: Kimi K2.5 model
   # - name: kimi-k2.5