Feature: Strongly type Python events

Hi,

I was trying to build against the SDK in Python, and I got confused about which events carried what payloads. The Data class is a union of all possible data fields:

https://github.com/github/copilot-sdk/blob/f7fd7577109d64e261456b16c49baa56258eae4e/python/copilot/generated/session_events.py#L1773

SessionEvent largely a pair of enum and Data with a long docstring:

https://github.com/github/copilot-sdk/blob/f7fd7577109d64e261456b16c49baa56258eae4e/python/copilot/generated/session_events.py#L3229

Other bindings in dotnet, for example, strongly type the events, so you know what events carry what payloads:

https://github.com/github/copilot-sdk/blob/f7fd7577109d64e261456b16c49baa56258eae4e/dotnet/src/Generated/SessionEvents.cs#L134

I think stronger type hinting on the Python side would help make the SDK easier to consume by catching classes of bugs at IDE/Pyright/agent coding time. It has precedent on the Claude Agent SDK, which does adopt this pattern for its message type:

https://github.com/anthropics/claude-agent-sdk-python/blob/13f65c81e2bec2a5d1f1e1efdd45aa061c4ec43f/src/claude_agent_sdk/types.py#L1071

It's a breaking change to people using type hints, but I suspect we could do it in such a way that the only breaks would be people genuinely looking in the wrong place for payloads (i.e. silent bugs).

从 python/copilot/generated/session_events.py 中的 Python Data 和 SessionEvent 定义开始，然后将 dotnet/src/Generated/SessionEvents.cs 中的强类型事件结构与对应的 Claude Agent SDK 类型进行比较。定义每个事件如何映射到其 payload 类型，并验证 Python 类型检查器能够为每个事件公开正确的 payload，而不依赖 union。

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