Oh, so no "own contribution" in the intentional sense...
If it had been in the construction drawings/cross-sections and also labeled as provided by the builder, we would have noticed it, asked about it, and prepared for it. The nice 3D and elevation drawings are irrelevant; what counts is what is stated in the scope of work description. And if it is not explicitly mentioned there, in the end it will not be executed. Yes, I share some of the blame with the general contractor - he should have at least pointed it out to us in advance. Among other reasons, clients turn to specialist companies because they [understandably] rarely have the technical qualification and knowledge themselves of what belongs to the complete work - and often cannot differentiate what is important or just aesthetics. Anyway, the die is cast, and we don’t need to discuss it further here.
There are such wedge elements for windowsills. At least they are hard, and in the past people have liked to cover up the problems you are having right now with them.
Thank you very much for the example, now I understand it too.