[10.x] Fix DB::afterCommit() broken in tests using DatabaseTransactions

[oprypkhantc]

Closes #50066

Currently, committing the "root" transaction causes DatabaseTransactionsManager to remove all $pendingTransactions. This tells the DatabaseTransactionsManager that it's outside of the transaction and makes it immediately execute callbacks on calls to DB::afterCommit():

class Something {
	public function something(): void {
		// DatabaseTransactionsManager::$pendingTransactions->count() === 0
		
		DB::transaction(function () {
			// DatabaseTransactionsManager::$pendingTransactions->count() === 1
			
			// Not executed immediately
			DB::afterCommit(fn () => dump(123));
		});

		// DatabaseTransactionsManager::$pendingTransactions->count() === 0
		//
		// dump(123) executed at this point
		
		// Executed immediately because of $pendingTransactions->count() === 0
		DB::afterCommit(fn () => dump(456));
	}
}

Outside of tests, this works as expected.

In tests that use DatabaseTransactions trait however, this causes an issue whenever you have a commit inside of your test:

class SomeTest extends TestCase {
	use DatabaseTransactions;

	public function testSomething(): void
	{
		// DatabaseTransactionsManager::$pendingTransactions->count() === 1
		
		DB::transaction(function () {
			// DatabaseTransactionsManager::$pendingTransactions->count() === 2
			
			// This will only execute after the transaction commit, as expected
			DB::afterCommit(fn () => dump("after commit"));
		});
		
		// DatabaseTransactionsManager::$pendingTransactions->count() === 0
		//
		// Here the $pendingTransactions no longer contains the "root" transaction, 
		// causing subsequent `afterCommit()` calls to immediately execute:

		DB::transaction(function () {
			// DatabaseTransactionsManager::$pendingTransactions->count() === 1
			
			// Unlike the DB::afterCommit() a few lines above, this one is executed immediately,
			// without waiting for the transaction to commit, which is not expected
			DB::afterCommit(fn () => dd(123));
		});
	}
}

This PR fixes this by not letting commit() remove a pending transaction of a lower level than what's being committed. The result is that committing a nested transaction in tests leaves the root $pendingTransactions in place.

Please let me know if we need more tests or explanations for this. Feel free to make changes on the branch.

[driesvints]

Thanks @oprypkhantc. Appreciate the PR.