NesXtabHistoryTracker stores many copies of really large documents

[roblourens]

I took a heap snapshot of the EH after using chat for awhile. The heap was 2.1GB. Copilot analyzed the snapshot and said this

The largest retained self-size group is 19 giant string nodes totaling about 699.7 MB. Each string is about 37 MB and starts with a Copilot agent session transcript JSONL payload (`session.start`, `session.mode_changed`, `user.message`, etc.).

The representative string node retainer path is:

- 37 MB string containing the session transcript
- retained by an object property `value`
- retained by an object property `base`
- retained by an object property `edit`
- retained by arrays under property `xtabEditHistory`

This points to `NesXtabHistoryTracker`, which was bounded by entry count only (`MAX_HISTORY_SIZE = 50`) but each history entry could retain a full `StringText` base or visible-range document content. A large JSONL/log document edited while inline-edit history tracking is active can therefore be retained multiple times. In this snapshot, 19 history entries retained about 700 MB of duplicate large document text.

I had opened a really large events.jsonl from copilot and it seems like that is causing us to use a ton of memory in the EH