[Mysql][SingleStore]: fix dangling event listeners in iterator()

[sijie-Z]

Summary

Fixes #5839

Problem

The iterator() method in both mysql2 and singlestore session implementations leaves dangling once() event listeners on the underlying stream. When the generator exits (normally or via error), once(stream, 'end') and once(stream, 'error') listeners remain attached. Per-iteration stream.once('data', resolve) listeners also leak when another branch of Promise.race wins.

In long-running applications, this causes MaxListenersExceededWarning and memory growth proportional to iterator call count.

Solution

• Use AbortController to register once() listeners with a signal, then call ac.abort() in the finally block to remove them
• Track per-iteration data listener references and clean them up at the start of each loop iteration before registering a new one

Applied identically to both drizzle-orm/src/mysql2/session.ts and drizzle-orm/src/singlestore/session.ts.

Testing

Existing iterator tests (select iterator, select iterator w/ prepared statement) cover the happy path. The fix is a resource cleanup change — no behavior change for consumed iterators, only prevents listener accumulation on stream exit.

[Nexory]

The AbortController + signal part closes the onEnd and onError leaks cleanly — that's the bulk of the listener accumulation.

One residual gap on the final iteration: when the loop exits via the break path (onEnd wins the last Promise.race, row resolves to undefined/empty) or via throw (onError wins), execution jumps straight to finally without re-entering the top of the loop. At that point dataResolve still holds the stream.once('data', resolve) from the last iteration. That listener was registered with no signal, so ac.abort() doesn't touch it, and the finally block only removes the permanent dataListener. So exactly one 'data' listener leaks per iterator() call — MaxListenersExceededWarning will still accumulate, just at roughly one-third the original rate.

Mirroring the per-iteration cleanup pattern into finally should close it:

} finally {
    ac.abort();
    stream.off('data', dataListener);
    if (dataResolve) {
        stream.off('data', dataResolve);
    }
    if (isPool(client)) {
        conn.end();
    }
}

Same change in both mysql2/session.ts (~line 202) and singlestore/session.ts (~line 201).

Happy to send a follow-up PR with this plus a unit-level reproduction that pins listenerCount on a mocked EventEmitter stream.