Fix disposed DbContext issue (#15090)

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #15090

Description

The InitializeMemberApplicationNotificationHandler is invoked at start-up - also during install. Problem is... it has an indirect dependency on UmbracoDbContext, which ultimately causes the linked issue in rare cases (it's a timing thing).

This PR ensures that InitializeMemberApplicationNotificationHandler resolves the IMemberApplicationManager at runtime instead of through the DI, which in turn ensures that the handler does not have a DI bound dependency on UmbracoDbContext.

Furthermore we're now throwing an exception in UmbracoDbContext if the connection string has not yet been set. This should prevent these kinds of issues from escalating in the future.

The exception throwing in UmbracoDbContext also means that UmbracoPooledDbContextFactory is no longer necessary. This was initially introduced as an attempted workaround to the same root cause.

Testing this PR

It should be possible to install V12.3.4 😄

Install should be performed with and without MemberAuthorization configured for the Delivery API (see docs):

• When configured, the umbracoOpenIddictApplications table should reflect the configuration after install.
• When not configured (or not enabled), the table should be empty.

[niekvanderreest]

Hi,

I'm running into "Connection string changed, disposing context" warnings after upgrading a project from umbraco 11.5 to 12.3.6, i don't see any errors and functionality seems to be what is was, but the warnings occur running an async task on and efCoreScopeProvider scope.

The project has Hangfire running on the umbraco database, the warnings occur while running a Hangfire job

Any idea if this is related, i would like to know the impact before pushing my upgrade to live (project is hosted on Umbraco Cloud)