feat: 支持通过 GenerateOptions 按轮次动态控制 enableThinking

[Janemia]

改动说明

为 GenerateOptions 新增 enableThinking 字段，使 Hook 能够按轮次动态控制 enable_thinking（此前只能控制 thinking_budget）。

修改内容

• GenerateOptions：新增 enableThinking 字段、getter、Builder 方法及 mergeOptions 合并逻辑
• DashScopeChatModel.applyThinkingMode()：优先取 options 级别的值，未设置时 fallback 到模型级别的固定值

向后兼容

完全向后兼容。当 options.getEnableThinking() 为 null 时，行为与改造前一致，使用模型级别的 this.enableThinking。

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[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...a/io/agentscope/core/model/DashScopeChatModel.java	71.42%	1 Missing and 1 partial ⚠️
...java/io/agentscope/core/model/GenerateOptions.java	83.33%	0 Missing and 1 partial ⚠️

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[LearningGp]