test(transactions): demonstrate ambient tx loss across await in mutate()

[marius-se]

🎯 Changes

Adds a test that demonstrates a mismatch between Transaction#mutate's documented contract and its implementation for async callbacks.

The contract

The docstring on Transaction#mutate (packages/db/src/transactions.ts:248-251) states:

If the callback returns a Promise, the transaction context will remain active until the promise settles, allowing optimistic writes after await boundaries.

What actually happens

mutate() invokes the callback synchronously and pops the transaction off the ambient stack in a finally block (packages/db/src/transactions.ts:287-298):

mutate(callback: () => void): Transaction<T> {
  // ...
  registerTransaction(this)
  try { callback() }                  // not awaited
  finally { unregisterTransaction(this) }
  // ...
}

When the callback is async, callback() returns its Promise at the first await, the finally fires immediately, and unregisterTransaction(this) removes the transaction from transactionStack before any post-await code runs. Collection operations after the await see no ambient transaction and either throw MissingInsertHandlerError (no onInsert) or create a brand-new direct transaction via the collection's handler.

Additional signals that the async path is unsupported today:

• The callback parameter is typed () => void, not () => void | Promise<void>.
• No existing test in packages/db/tests/ exercises an async mutate() callback. The async tests in transactions.test.ts cover mutationFn and commit(), not the mutate() callback.

✅ Checklist

• I have tested this code locally with pnpm test.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).