refactor(model): integrate ModelAPI into model proxy and service classes

[OhYee]

This change introduces ModelAPI as a mixin to both ModelProxy and ModelService classes, consolidating common API functionality. The completions and responses methods have been removed from these classes as they are now inherited from ModelAPI, reducing code duplication and improving maintainability.

The refactoring affects multiple files including __model_proxy_async_template.py, __model_service_async_template.py, model_proxy.py, and model_service.py, along with the new model_api.py module definition.

Two new files were created: agentrun/model/api/model_api.py which defines the ModelAPI base class, and examples/embedding.py which provides usage examples for embedding functionality.

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此更改将ModelAPI作为mixin引入到ModelProxy和ModelService类中，
整合了通用的API功能。completions和responses方法已从这些类中移除，
因为它们现在继承自ModelAPI，减少了代码重复并提高了可维护性。

重构影响了多个文件，包括__model_proxy_async_template.py、
__model_service_async_template.py、model_proxy.py和model_service.py， 以及新的model_api.py模块定义。

创建了两个新文件：agentrun/model/api/model_api.py定义了ModelAPI基类， examples/embedding.py提供了嵌入功能的使用示例。

Change-Id: I81045039bef65df9d7daab6a56ff2e1442fddcb7

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