Cursor compact reload restores full toolResult bodies so the retry still resource_exhausted

[leeseunguk]

Summary

Native Cursor (cursor / cursor-cli-oauth) rejects long sessions with 0-token resource_exhausted because the wire payload still contains verbatim toolResult bodies (often hundreds of KB–MB). Compaction does not fix this:

1. findCutPoint cannot cut at toolResult, so the last tool turn is kept whole.
2. After a successful compact, AgentSession replaces agent.state.messages from sessionManager.buildSessionContext(), which reloads those full bodies from jsonl.
3. The retry then sends the same payload. A second compact often throws Nothing to compact (session too small) and the TUI shows Pre-prompt compaction failed / Retry failed after 2 attempts.

Mid-turn compact skip (#984) is still correct (it poisons conversationId). The missing piece is Cursor-only truncation of toolResult text at admission and again after compact reload, plus remint.

Evidence (session 01a01a3e)

• Compact reported tokensBefore: 99166 then saved ~11%; Cursor still RE.
• jsonl toolResult bodies stayed >2k chars (66 of them); truncation was in-memory only and wiped on reload.
• After overflow compact 31818 → 4306 on the session.log meter, jsonl still had the original bodies.

Proposed fix

• Export truncateToolResultBodies(messages, 2000) and apply it on Cursor compactBeforeNextAdmission before the Cursor compaction during a live turn poisons conversationId #984 skip return, returning true when messages changed so prepareNextTurn uses agent.state.messages.
• Call the same helper after compact applies sessionContext.messages and in _restoreAgentMessagesFromSession.
• Remint (allowConversationRotate) when truncated.
• Do not persist truncated bodies to jsonl; do not change x.ai / other providers.

Why not an extension

Admission and compact reload are private AgentSession paths. There is no hook after buildSessionContext() replaces agent messages.