[12.x] Fix model pruning when non model files are in the same directory

[rojtjo]

The changes introduced in #56060 broke the model:prune command since it doesn't check if the given class is actually an Eloquent model before calling one of it's methods. This PR fixes that by adding a simple type check while still using the newly introduced isPrunable method to perform the check.

Please note that for the most part this PR is setup code to allow testing for the error.

Related: #56068

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[rojtjo]

We might also want this check in this section

framework/src/Illuminate/Database/Console/PruneCommand.php

Lines 87 to 108 in c86346f

	/**
	* Prune the given model.
	*
	* @param string $model
	* @return void
	*/
	protected function pruneModel(string $model)
	{
	$instance = new $model;
	$chunkSize = property_exists($instance, 'prunableChunkSize')
	? $instance->prunableChunkSize
	: $this->option('chunk');
	$total = $model::isPrunable()
	? $instance->pruneAll($chunkSize)
	: 0;
	if ($total == 0) {
	$this->components->info("No prunable [$model] records found.");
	}
	}

, but that seems to mess up the error messages defined in the other tests. Thoughts?