Server side markdown parser turns tabs inside code blocks into spaces

If you send the message:

```
real	0m0.322s
user	0m0.344s
sys	0m0.036s
```

it turns into:

```
real 0m0.322s
user 0m0.344s
sys 0m0.036s
```

when it goes through the server-side markdown renderer. The local echo looks fine.

Update by @alya 2025-01:

See https://github.com/zulip/zulip/issues/17419#issuecomment-2347094137 for specific action items to work on here.