Synapse responds 204 to `OPTIONS` regardless of the endpoint actually existing

### Description

When Synapse is approached with an `OPTIONS` request, it responds 204, always. It should be returning `404 M_UNRECOGNIZED` for endpoints which don't exist at all, and `405 M_UNRECOGNIZED` for non-client-server endpoints which do exist, as per the `M_UNRECOGNIZED` description at https://spec.matrix.org/v1.10/client-server-api/#common-error-codes

Synapse's current behaviour can cause endpoint discovery to be unreliable. For example, https://github.com/matrix-org/matrix-react-sdk/pull/12492 aims to use `OPTIONS` to discover sliding sync, but because Synapse *always* returns a success code, the react-sdk thinks sliding sync is supported (even though it's not, as of writing).

### Steps to reproduce

* `curl -sv -X OPTIONS https://matrix-client.matrix.org/_matrix/client/this/path/does/not/exist`

### Homeserver

all

### Synapse Version

at least 1.105.1

### Installation Method

Other (please mention below)

### Database

n/a

### Workers

I don't know

### Platform

n/a

### Configuration

_No response_

### Relevant log output

```shell
n/a
```

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

_No response_