fix(openai-compat): require a live conversation id before sending the tool reminder

[jailbirt]

The problem

schemasAlreadyInThread gates the resume-turn reminder on !needsCreate, which only says the session is present in the manager's map. That is weaker than it looks:

• start() (base-oneshot-session.ts:103) resolves the cwd, assigns a session id, sets _isReady and emits ready. It does not spawn anything.
• The conversation id is captured later — codex on thread.started (persistent-codex-session.ts:220), agy by harvesting the log after the first turn.
• A send that fails before that point does not remove the session from the map.

So after a failed first send, the next turn sees sessionExists = true, takes the resumed-thread path, and sends buildToolReminderBlock() — a reminder about tools the engine has never received. No schemas, no identity, no history. The request returns 200 with whatever the model makes of it.

It is a quiet failure: nothing in the logs distinguishes it from a healthy resume, which is why it took a while to find in our deployment.

The change

Require the id to actually be there:

if (!needsCreate && !perMessageMode && engineHasNativeConversation(engine)) {
  try {
    schemasAlreadyInThread = nativeThreadIsLive(engine, manager.getStatus(sessionName).stats);
  } catch {
    schemasAlreadyInThread = false;
  }
}

nativeThreadIsLive() is exported as a small pure predicate so the rule can be tested directly rather than through handleChatCompletion, which has no coverage today.

Both failure directions are safe by construction: when in doubt it sends the full block, which is correct but larger, never the reminder-without-schemas.

SessionManagerLike.getStatus declared its return shape by hand and left both ids out; they are added as optional, which is what they are on SessionStats.

Behaviour

situation	before	after
thread live (the normal case)	reminder	reminder — unchanged
session in map, id never captured	reminder about tools the engine never saw	full block
perMessageMode, fresh session, one-shot engines	full block	full block — unchanged

Tests

Six cases on the predicate: id required for codex / codex-app / agy, the two ids not interchangeable, and engines that hold context in a live process still treated as live so claude does not regress.

npm run build, npm run lint, npm run format:check clean; vitest run green at 900/900 across 58 files.

Why this one first

We run this in production on the codex engine (~15 agents) and have two more changes queued that both need a trustworthy "is the thread really there" predicate — trimming already-sent tool results, and skipping the system prompt on resumed turns. Both are unsafe on top of !needsCreate alone, so this seemed like the right thing to send first and on its own.

Separately filed #75, on contextPercent and the auto-compaction gate — not bundled here since it is a different area.