[12.x] Fix double query in model relation serialization

[AndrewMast]

Why

When a model has an eager-loaded relation through $with, the unserialization process ends up loading the relation twice (first through $with and then again when loading the relations that were serialized). Issue #55546 highlights what this PR tries to fix.

I created a repo that reproduces this error: https://github.com/AndrewMast/laravel-testing/tree/issue-55546 (see diff).

What

The restoreModel() method in SerializesAndRestoresModelIdentifiers.php uses the load method to fetch all the serialized relations. However, this leads to the double query for relations already loaded (i.e. in $with). Changing load to loadMissing resolves this issue.

[taylorotwell]

Tests failing.

[AndrewMast]

I will investigate the tests. I also will add a test to prevent double loading relations on unserialization.

[AndrewMast]

@taylorotwell I found the reason for this failure. When running the tests in the failing file (ModelSerializationTest.php) by itself, no error was raised. Only when running all of the tests together is there a failure. I found that this was due to the status of Model::preventsLazyLoading() changing depending on whether ModelSerializationTest.php was run by itself or not.

This could be due to tests/Integration/Http/ThrottleRequestsTest.php, tests/Integration/Http/ResourceTest.php, or tests/Integration/Database/EloquentStrictLoadingTest.php running Model::preventLazyLoading() or Model::shouldBeStrict().

I was able to fix the tests by adding Model::preventLazyLoading(false); to the setUp method of ModelSerializationTest.php.

I am currently adding a test for the issue this PR resolves, will mark as ready when done.

[AndrewMast]

Added a test utilizing expectsDatabaseQueryCount to ensure only 4 queries are called during unserialization (one for the model, three for the relations).