Track exec JSON compaction-events fix in commit 942131e8c00d5648deebadd353a38c54a9b85286

Summary

Tracking the fix in commit 942131e8c00d5648deebadd353a38c54a9b85286, which emits context compaction items in exec JSON output.

Fix commit:

• https://github.com/openai/codex/commit/942131e8c00d5648deebadd353a38c54a9b85286

Commit message:

• Emit compaction items in exec JSON output

What this changes:

• emits ContextCompaction thread items in exec JSON output
• maps legacy ContextCompacted notifications into completed compaction items
• adds tests covering both item notifications and the legacy notification path

Related context:

• #10986 discusses compaction visibility/regression, but does not mention this commit hash

Opening this issue so the fix is discoverable from the issue tracker by commit SHA.