Coerce missing billable_units to null on MCP run payloads

Nobody has claimed this yet.

## Summary

`billable_units` is forwarded from `result.billableUnits` without coercion in `lib/runs/execute.ts`. When that value is `undefined` (non-blocking/queued executions or older gateway responses), `JSON.stringify` omits the key even though the intended contract is `number | null` and the field should always be present.

Caught on `main` by a follow-up review of code that has not changed since the last PR review.

## Location

`lib/runs/execute.ts` (~line 252):

```ts
billable_units: result.billableUnits,
```

Contract tests already assert the field is present as `number | null`:

- `tests/contracts/run-execution.test.ts` — `"never repeats paid execution when terminal storage fails"` expects `billable_units` to be `null`
- `"forwards billable_units from the gateway result"` expects a numeric value when the gateway provides one

There is no coverage for the missing/`undefined` case.

## Proposed fix

Coerce missing units to `null` before putting them on the payload, e.g. `result.billableUnits ?? null`.

Add a contract test where the gateway result omits `billableUnits` (queued / non-blocking / older response) and assert the JSON payload still includes `"billable_units": null`.

## Test plan

- [ ] Coerce `undefined` / omitted `billableUnits` to `null` in the execute payload
- [ ] Keep forwarding numeric values unchanged
- [ ] Add a contract test for the omitted-units path

Start in lib/runs/execute.ts around the billable_units assignment, then read tests/contracts/run-execution.test.ts and run its existing contract tests. Add coverage for an omitted result.billableUnits value while preserving the numeric case. Done means the JSON payload always includes billable_units, using null when the gateway value is missing.

Written by the indexing model from the issue text.