rc_joins rate limits dont function

This issue has been migrated from [#16002](https://github.com/matrix-org/synapse/issues/16002).

---

### Description

For test purposes i added this to my homeserver yaml:
```

rc_joins:
local:
per_second: 1000
burst_count: 2000
remote:
per_second: 1000
burst_count: 2000
```

but i still can just join 10 rooms and then get the rate limit error

### Steps to reproduce

change config and try to join a bunch of rooms at once

### Homeserver

my own

### Synapse Version

1.88.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

Postgres 15

### Workers

Single process

### Platform

Docker

### Configuration

see discription

### Relevant log output

```shell
MatrixError: [429] Too Many Requests
```

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

_No response_

Start by reproducing the 429 response with the reported rc_joins configuration on Synapse 1.88.0, then trace the room-join rate-limit handling from that configuration entry point. Done means the configured local and remote limits are honored, with a regression test covering the reported setup.

Written by the indexing model from the issue text.