Mocking support

Due to the design of the GenAI Client() class it isn't possible to mock it for most use cases.

For example (with Mockito):

```
Client clientMock = mock(Client.class);
when(clientMock.models).thenReturn(mock(Models.class));
```
A simple scenario such as this won't work because the `models` property in Client is final and can only be set through constructors that I also can't override because the class is marked as final. It might be possible to use `mockito-inline` but this isn't ideal for a number of reasons.

The idiomatic way for the Client class to be defined in Java is for models to be accessed via a getter which would also support basic mocking but other ways to support mocking a Client would be welcome.