Cover scenario `BEGIN TRANSACTION; COMMIT; BEGIN TRANSACTION; --failure`

[simPod]

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Q	A
Type	improvement
Fixed issues

Summary

Covers the concern expressed by @greg0ire https://github.com/doctrine/dbal/pull/6545#issuecomment-2412377607

Transaction should not be rolled back in autocommit=false mode when BEGIN TRANSACTION fails due to network error or similar after COMMIT.