/createRoom API behaviour does not match spec

### Background information

- **Dendrite version or git SHA**: docker main
- **SQLite3 or Postgres?**: sqlite3
- **Running in Docker?**: yes
- **`go version`**: docker main
- **Client used (if applicable)**: matrix-js-sdk

### Description

- **What** is the problem: /createRoom API uses join_rule=invite if preset is unspecified even if visibility is specified
- **Who** is affected: Unclear
- **How** is this bug manifesting: Creating rooms unlike the intent of the creator
- **When** did this first appear: Unclear

https://github.com/element-hq/dendrite/issues/3123#issuecomment-2575539315 for more context

The spec does say to use `visibility` if `preset` is unspecified.

> If unspecified, the server should use the visibility to determine which preset to use. A visibility of public equates to a preset of public_chat and private visibility equates to a preset of private_chat.

https://spec.matrix.org/v1.13/client-server-api/#post_matrixclientv3createroom

### Steps to reproduce

- call /createRoom API with visibility=public
- receive a room via /sync which has join_rule=invite instead of join_rule=public

Start at the /createRoom API handling and reproduce the request with visibility=public and no preset. Compare the resulting join_rule with the Matrix specification's visibility-to-preset behavior; done means a public visibility request produces the intended public room configuration without an explicit preset.

Written by the indexing model from the issue text.