refactor(ark): remove explicit tool choice assignment from chat model (#695)

Removed explicit `toolChoice` field assignment in tool binding methods
for both standard ChatModel and ResponsesAPIChatModel.

Changes:
- Remove `toolChoice = &tc` assignment in WithTools(), BindTools(), and
  BindForcedTools() methods in chatmodel.go
- Remove `toolChoice` assignment in ResponsesAPIChatModel.WithTools()
  method in responses_api.go
- Simplify tool binding logic by relying on default behavior instead of
  explicit ToolChoiceAllowed/ToolChoiceForced settings

This change simplifies the codebase by removing redundant tool choice
configuration, allowing the underlying implementation to handle tool
choice behavior implicitly.

Author: JonXSnow

components/model/ark/chatmodel.go

@@ -462,18 +462,12 @@ func (cm *ChatModel) WithTools(tools []*schema.ToolInfo) (fmodel.ToolCallingChat
 		return nil, fmt.Errorf("failed to convert to ark responsesAPI tools: %w", err)
 	}
 
-	tc := schema.ToolChoiceAllowed
-
 	ncm := *cm.chatModel
 	ncm.rawTools = tools
 	ncm.tools = arkTools
-	ncm.toolChoice = &tc
-
 	nrcm := *cm.respChatModel
 	nrcm.rawTools = tools
 	nrcm.tools = respTools
-	nrcm.toolChoice = &tc
-
 	return &ChatModel{
 		chatModel:     &ncm,
 		respChatModel: &nrcm,
@@ -484,23 +478,16 @@ func (cm *ChatModel) BindTools(tools []*schema.ToolInfo) (err error) {
 	if err = cm.tryBindTools(tools); err != nil {
 		return err
 	}
-
-	tc := schema.ToolChoiceAllowed
-	cm.chatModel.toolChoice = &tc
-	cm.respChatModel.toolChoice = &tc
-
 	return nil
 }
 
 func (cm *ChatModel) BindForcedTools(tools []*schema.ToolInfo) (err error) {
 	if err = cm.tryBindTools(tools); err != nil {
 		return err
 	}
-
 	tc := schema.ToolChoiceForced
 	cm.chatModel.toolChoice = &tc
 	cm.respChatModel.toolChoice = &tc
-
 	return nil
 }
 

components/model/ark/responses_api.go

@@ -1608,11 +1608,9 @@ func (cm *ResponsesAPIChatModel) WithTools(tools []*schema.ToolInfo) (model.Tool
 	if err != nil {
 		return nil, fmt.Errorf("failed to convert to ark responsesAPI tools: %w", err)
 	}
-	tc := schema.ToolChoiceAllowed
 	nrcm := *cm
 	nrcm.rawTools = tools
 	nrcm.tools = respTools
-	nrcm.toolChoice = &tc
 
 	return &nrcm, nil
 }