fix(sdk): re-dispatch a single in-flight user on recovery boot (#4768)

<!-- ccr-slack-attribution -->
_Requested by **Matt Aitken** · [Slack
thread](https://triggerdotdev.slack.com/archives/C061L2MHW93/p1787615162456839?thread_ts=1787615162.456839&cid=C061L2MHW93)_

**Before:** a `chat.agent` run is killed mid-answer (OOM, crash,
eviction) while the message it was answering is the only one still
outstanding. The new run boots, puts that message and the half-written
reply into its context, and then waits for a message that already
arrived. Nobody ever answers the user; the run sits idle until it times
out.

**After:** the new run re-runs that message as a fresh turn and replies
to it. The half-written reply is dropped. When two or more messages are
outstanding, nothing changes — the interrupted one still goes into
context and the newer ones are re-run, exactly as before.

## ✅ Checklist

- [x] I have followed every step in the [contributing
guide](https://github.com/triggerdotdev/trigger.dev/blob/main/CONTRIBUTING.md)
- [x] The PR title follows the convention.
- [x] I ran and tested the code works

---

## Testing

New regression test in `packages/trigger-sdk/test/recovery-boot.test.ts`
— seeds a partial assistant plus exactly one in-flight user, no
`onRecoveryBoot`, and asserts one turn fires for that user with the
orphan partial dropped from the chain. It fails on `main` (`turnCount`
0, no turn at all) and passes with this change.

- `pnpm exec vitest run` in `packages/trigger-sdk` — 373 passed, 1
skipped (31 files passed, 1 skipped)
- `pnpm exec oxfmt --check` on the changed files — clean
- `pnpm exec oxlint packages/trigger-sdk/src packages/trigger-sdk/test`
— clean
- `pnpm run build --filter @trigger.dev/sdk` — clean

**What it does:** with exactly one in-flight user on a recovery boot,
re-dispatch that user as a fresh turn instead of splicing it into the
seed chain, where it was never answered.

**How:** the recovery-boot smart default made one decision in two halves
— the seed chain and the recovered-turn list — both gated on
`partialAssistant !== undefined && inFlightUsers.length > 0`. The splice
consumes `inFlightUsers[0]` into the chain as "the question the partial
was answering" and dispatches the rest. That only works when there *is*
a rest: at n=1 `recoveredTurns` came out empty, the boot-injected queue
stayed empty, the `session.in` cursor was advanced past the message
anyway, and on a `preload` or continuation boot (no `message` on the
wire payload) neither dispatch site fired. Both branches now require
`length > 1`, so n=1 falls through to the documented default — chain =
`settledMessages`, re-dispatch every in-flight user. The submit-message
boot is unaffected: the existing dedup still drops a queued message
identical to the one already on the wire payload.

Also corrected alongside it: the two SDK docstrings and the
`docs/ai-chat/patterns/recovery-boot.mdx` defaults section, which
described the default as "re-dispatch every user" and never mentioned
the splice.

Follow-up (not in this PR): the webapp e2e OOM helper never streams a
token before throwing, so it exercises the no-partial path only and
would not have caught this. Worth a variant that emits a token first.

---

## Changelog

Fixed a chat agent hanging after an interrupted turn: when a run was
killed mid-answer and only the one message it was answering was still
outstanding, the new run never replied to it. That message is now
re-answered on the new run.

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Matt Aitken <matt@mattaitken.com>
Co-authored-by: Eric Allam <eric@trigger.dev>