Copilot App cannot reopen a session after events.jsonl grows beyond 560 MB

### Short summary

A long-running Copilot App session becomes unavailable after its history log grows too large.

### Affected version or release

v1.1.12 on windows

### Installation context

Locally installed GitHub Copilot App. One local repository session was affected.

### What happened?

The session remained visible in the App, but opening/resuming it failed with:

RPC error -32603: Invalid string length

The session metadata and events.jsonl file were otherwise intact. The file size was approximately 561 MB.

### Steps to reproduce

1. Use a long-running session with large tool outputs.
2. Allow events.jsonl to grow to approximately 561 MB.
3. Close the Copilot App.
4. Try to reopen or resume the session.
5. Observe that the session is listed but cannot be opened.

### Expected behavior

The App should reopen large sessions, or provide a recoverable compaction/error flow instead of making the session inaccessible.

### Additional context

Workaround: I stopped the session, backed up events.jsonl, and reduced it from approximately 561 MB to 27.6 MB by removing older history while retaining the initial events and latest compacted history. The session opened successfully after replacing the file.

No other session metadata changes were required.

使用 events.jsonl 大约为 561 MB 的会话重现该故障，然后跟踪返回 RPC 错误 -32603: Invalid string length 的重新打开/恢复流程。将其与备份的 27.6 MB 文件进行比较，并将以下情况定义为完成：能够重新打开大型会话，或提供无需手动编辑历史记录的可恢复压缩/错误流程。

由索引模型根据 Issue 内容生成。