Messages sent out of order.

### Steps to reproduce

1. On bad connectivity, i sent 5 messages in a row.
2. The 2nd one got a sent receipt before the 1st one.
3. It transpires the 2nd one got sent first, thus out of order.

### Outcome

#### What did you expect?

Messages should always be sent in the order they were written.

In other words, the client should wait for a 200 OK from the first send before it attempts the second send.

#### What happened instead?

Message send order regressed again - see also https://github.com/element-hq/element-web/issues/22885 and https://github.com/element-hq/element-web/issues/6398 and https://github.com/element-hq/element-web/issues/11534 and https://github.com/matrix-org/matrix-js-sdk/pull/3131 and https://github.com/element-hq/element-web/issues/18942 etc. etc.

See logs & screenshot of what happened.

### Operating system

_No response_

### Application version

ElementNightly/2025040301

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

Yes