AssertionError: Requester must have an access_token_id when sending room events with MAS personal access tokens

### Description

Sending room events with a MAS personal access token (created via the MAS admin API `/api/admin/v1/personal-sessions`) results in a 500 with `M_UNKNOWN Internal server error`.

The client receives no useful error and Synapse logs show:

```
AssertionError: Requester must have an access_token_id
File "synapse/rest/client/transactions.py", line 98, in _get_transaction_key
```

### Steps to reproduce

1. Synapse configured with MAS delegated auth
2. Create a MAS personal session via `POST /api/admin/v1/personal-sessions` with scope `urn:matrix:org.matrix.msc2967.client:api:*`
3. Use the returned token to `PUT /_matrix/client/v3/rooms/{roomId}/send/m.room.message/{txnId}`
4. Get a 500, `AssertionError: Requester must have an access_token_id`

### Homeserver

private homeserver

### Synapse Version

1.149.1+ess.1

### Installation Method

Other (please mention below)

### Database

PostgreSQL 17.9

### Workers

Single process

### Platform

K8s via https://github.com/element-hq/ess-helm

### Configuration

_No response_

### Relevant log output

```shell
AssertionError: Requester must have an access_token_id
File "synapse/rest/client/transactions.py", line 98, in _get_transaction_key
```

### Anything else that would be useful to know?

_No response_