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.

Reproduce the failure with a session whose events.jsonl is approximately 561 MB, then trace the reopen/resume flow that returns RPC error -32603: Invalid string length. Compare it with the backed-up 27.6 MB file and define done as reopening large sessions or providing a recoverable compaction/error flow without manual history edits.

索引モデルが issue の本文から書いたものです。