Hybrid Cache: Resolve start-up errors with mis-matched types (closes #20537)

[AndyButland]

Prerequisites

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

Addresses: #20537

Description

This PR makes two updates to resolve the linked issue;

• Ensure we use a nullable type when checking for existence to align with retrieving from or creating in the cache. As were using a nullable type for one and not the other, this could explain why the hybrid cache sees misaligned types but reports them as the same.
• Ensure we use the same overload for checking existence and retrieving from or creating in the cache. Possibly there's an issue here with the incorrect state object being applied meaning the types are considered different.

For background, this "exists" check was added in #19890 which was introduced to improve start up performance (allowing us to retrieve cached documents in batches from the database rather than one at a time).

I was able to reproduce the problem manually - it seemed to be consistent at least when using the Visual Studio template and IIS Express:

• Create a new Umbraco project using 16.3.1.
• Create a document type with a property, a template and a root content item.
• Start Umbraco via Ctrl + F5 using IIS Express.
• The reported error message could be seen.

The amends described above resolved the problem with the hybrid cache exception but revealed something else - that we would get an immediate page not found, and then after a refresh, the expected templated page would show. The crux of the problem here seemed to be that we are doing seeding on an application started notification, which doesn't seem correct. We should seed the initial content before we accept requests, and as such I've moved this to an application starting notification. That looks to resolve this problem.

There are some integration tests added in an attempt to replicate the problem. These didn't succeed in doing so, but are probably worth keeping anyway as additional testing around the hybrid cache.

Similarly there's some tightening up around the implementation of the "exists" check (the locking around "check and remove" and the cancellation token handling). None of this turned out to be the smoking gun, but I think they are valid improvements to consider keeping.

Testing

First try to replicate the problem consistently on main and then when you can, switch in the code from this PR and verify that the application starts without error and the expected initial page is shown.

[Copilot]

Pull Request Overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

[nikolajlauridsen]

Looks good, and tests good to me too.

I was only able to reproduce by adding an actually redis cache, but then the issue also became very consistent, but this PR fixes it, great work 🎉

[nikolajlauridsen]

Cherry picked to release/17.0 👍

[umbracocommunity]

This pull request has been mentioned on Umbraco community forum. There might be relevant details there:

https://forum.umbraco.com/t/umbraco-upgrade-15-4-4-16-3-1-hybridcache-with-the-same-key/6363/4