Convert ProcessorTransactionLog into a trait.

[andriyDev]

Objective

• There was a TODO to make this type actually a trait.
• Add a few basic tests for asset processing. #21409 introduced a test flake on WIndows (since the transaction log file may not have been fully released by the time the next test ran), and made tests create the crates/bevy_asset/imported_assets/log file.

Solution

• Create two traits: ProcessorTransactionLogFactory and ProcessorTransactionLog. The former creates the latter.
• Rewrite the test helper for asset processing to use a fake transaction log that doesn't do anything.
• Note: pretty much the entire implementation was just reformatted into the trait.
• Note: these transaction log methods are returning BevyError instead of something like std::io::Error.

Testing

• I ran the asset tests on repeat for a while (including with Add tests for asset processing to show the problem with #21269. #21433 where the flake was first seen and I was able to reproduce fairly quickly). No issues!

Note: we could make a release notes for the fact that users can make their own transaction logs, but this feature is primarily for unit testing. Maybe a user could make a more robust transaction log, but it's really not clear that it would be important for anyone.

[janis-bhm]

Suggested change

	/// Sets the transaction log factory prior to processing being started.
	/// Replaces the default transaction log factory. If this is called after asset processing has begun in the `Startup` schedule, it will fail and return `SetTransactionLogFactoryError::AlreadyInUse`.

just a nit, but I think this could be clearer.

[andriyDev]

I expanded this with your inspiration. I don't want to say "replaces the default", since if you call this multiple times it'll be replacing your transaction log. I did however clarify not calling it will use the default log. I also don't want to be specific about what error you get, since there may be other errors, but I highlighted the startup schedule like you suggested.

[janis-bhm]

Is there a way to pass the factory via AssetPlugin and not have the setter at all?

[andriyDev]

Is there a way to pass the factory via AssetPlugin and not have the setter at all?

I initially didn't want to do this because it means putting a lot of processor specific config in the AssetPlugin, but I decided to give it a shot.

The problem is that Plugin::build is given an immutable borrow to the plugin, so we can't easily take a box for the factory. We'd need to have something like processor_transaction_log_factory: Mutex<Option<Box<dyn ProcessorTransactionLogFactory>>>, which is an awful type to work with.

[janis-bhm]

We'd need to have something like processor_transaction_log_factory: Mutex<Option<Box<dyn ProcessorTransactionLogFactory>>>, which is an awful type to work with.

Yeah that doesn't sound great. My hope was that the ambiguity of when the factory can be set could be turned into more of a type-thing rather than being a docs-thing, if that makes sense.