[sdk-platform-java] Client libraries should be mockable for testing

**Is your feature request related to a problem? Please describe.**
I am attempting to write JUnit tests for my code that integrates with Dialogflow CX. However, the `SessionsClient` does not implement a service interface, and the business methods (particularly `detectIntent(DetectIntentRequest)`) are all `final`. This means that it is impossible to use a test double, either a JDK interface proxy or a CGLib subclass.

**Describe the solution you'd like**
The service APIs should implement a business service interface that can be mocked.

**Describe alternatives you've considered**
No alternatives considered; this is fundamental baseline functionality for an integration library.

从 Dialogflow CX SessionsClient 入口点及其 detectIntent(DetectIntentRequest) 方法开始，然后审查服务 API 如何为 JUnit 测试提供。定义可模拟服务接口的范围，并验证客户端集成可以使用 JDK 接口代理或 CGLib 子类作为测试替身。

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