fix(dashscope): enhance support for native structured output in DashScope models

[chickenlj]

related to #1865,#1743,#1640

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[codecov]

Codecov Report

❌ Patch coverage is 37.14286% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...e/src/main/java/io/agentscope/core/ReActAgent.java	28.57%	9 Missing and 1 partial ⚠️
...a/io/agentscope/core/model/DashScopeChatModel.java	10.00%	8 Missing and 1 partial ⚠️
...core/formatter/dashscope/DashScopeToolsHelper.java	50.00%	1 Missing and 1 partial ⚠️
...s/model/openai/formatter/OpenAIResponseParser.java	85.71%	0 Missing and 1 partial ⚠️

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[itxaiohanglover]

Good robustness fix! The fallback from native to synthetic structured call prevents hard failures. The context cleanup on error (removing added messages, resetting nativeResponseFormat) is important for not corrupting state on retry.