Escape characters (backslashes) can cause bugged markup outputs

[Th-Underscore]

Describe the bug

When including backslashes in a specific arrangement around upticks, markup is generated incorrectly and is sent literally in chat.

Reproduction

1. Type:

   `\`test\``
   

   into the composer.

2. Send the message.

Expected behavior

In the provided repro case, it should apply code-block formatting onto the first backslash, then this section literally:

test``

It should look like:

Platform and versions

1. OS: Ubuntu 24.04.4 LTS
2. Opera Developer 134.0.5938.0
3. Sable versions tested on:
  - Desktop client: v1.20.0-dev (2041349)
  - Homeserved webapp: v1.20.0-dev (0865d93)
  - Official webapp: v1.20.0-dev (e3e1d57)
4. Matrix servers tested on:
  - matrix.org
  - My private Synapse server

Additional context

Doesn't happen with the live preview in #1757.