Cannot use password auth provider callbacks when OAuth delegation is enabled

**Description:**

I would like to use Matrix Auth Service for OAuth through Google while also maintaining an LDAP server of local users and passwords. This is [currently not supported](https://github.com/element-hq/synapse/blob/1920dfff40ad1078071e099a2afbfa31a5409e6b/synapse/module_api/__init__.py#L443).

Start by reading synapse/module_api/__init__.py at the linked callback restriction, then trace how OAuth delegation and password-auth provider callbacks are handled. Identify the relevant tests or entry points for both mechanisms; done means they can be enabled together while LDAP-backed password users continue to authenticate.

Written by the indexing model from the issue text.