[v2][Python] Make generated implementation modules private

https://github.com/github/copilot-sdk/blob/main/python/copilot/generated/__init__.py Makes it clear that the `copilot.generated` subpackage is not meant to be public:

https://github.com/github/copilot-sdk/blob/cd2b10b07e8f393b09379dfda6b30f658b2963d2/python/copilot/generated/__init__.py#L3

But the subpackage doesn't start with an underscore to signify it's an implementation detail: `copilot._generated`.

As well, https://github.com/github/copilot-sdk/blob/main/python/copilot/rpc.py and https://github.com/github/copilot-sdk/blob/main/python/copilot/session_events.py which are meant to act as the public API potentially over-expose things. While `copilot.rpc` does export something explicitly:

https://github.com/github/copilot-sdk/blob/cd2b10b07e8f393b09379dfda6b30f658b2963d2/python/copilot/rpc.py#L10-L11

... both files **also** do an `import *` followed by an import of `__all__` from the generated code which lists everything contained in the generated code. E.g.:

https://github.com/github/copilot-sdk/blob/cd2b10b07e8f393b09379dfda6b30f658b2963d2/python/copilot/session_events.py#L9-L10

exposes 330 symbols in the module:

https://github.com/github/copilot-sdk/blob/cd2b10b07e8f393b09379dfda6b30f658b2963d2/python/copilot/generated/session_events.py#L9883-L10215

I'm not sure if all 330 symbols are meant to be exposed, but at least `copilot.generated` should be made private by renaming it to `copilot._generated`. If all of those symbols are meant to be exposed then that one-liner in `copilot.rpc` might as well be removed. If they are not meant to be exposed then the `import __all__` and `import *` should go and you can explicitly set `__all__` in the exporting modules.

从 python/copilot/generated/__init__.py、python/copilot/rpc.py、python/copilot/session_events.py 以及生成的 rpc.py 和 session_events.py 模块开始。在更改包名或导出列表之前，追踪哪些生成的符号应当是公开的。完成的标准是实现模块为私有，且公共模块只公开预期的 API，不会意外公开生成的符号。

由索引模型根据 Issue 内容生成。