Toilet too close to the wall, what now?

Google recommends at least 20cm, but that all sounds more like a recommendation rather than a DIN standard or something similar. From a pure layman's gut feeling, that falls under "tough luck" (aka should have been noticed earlier) and if you really wanted to move the toilet, that sounds complicated :( but as said, it's just a layman's gut feeling.

where exactly am I supposed to know that the representation of tiles in the bathroom is supposed to be 30 cm tiles?

I went to secondary school before Pisa, my calculation is: 90 cm door and 90 cm window (both are dimensioned and three tiles wide) >> apply ratio rule >> they must be 30 cm tiles >> see that next to the toilet there is half a tile width distance to the wall >> derive a justified suspicion that it is probably around 15 cm distance;

And how am I supposed to know how much distance one normally has to the wall?

Know where the "measuring tape" is ;-)

It is a prefabricated house and all drawings and plans come from the house manufacturer

I've been saying that all along: whether someone else has checked something sufficiently, you don't know - so if in doubt, don't do it twice, but do it yourself first.

The 10cm grid also results from the underfloor heating panel. One should have been suspicious in terms of space requirements.

You simply can't fit everything into a 1.20m niche...

There is no DIN, but a standard

It is a prefabricated house and all drawings and plans come from the house manufacturer

Yes, like with so many. But for example, I put everything on graph paper and read up on missing knowledge, in other words, informed myself. There was also a bathroom in my apartment and there too there was potential to think about how much space is needed and how much not.

And how exactly am I supposed to know that the representation of tiles in the bathroom is supposed to be 30cm tiles?

You don’t know that, that’s true. However, a toilet is about 35cm wide, and then you can see that on the right side of the plan there is no more space than half the toilet width.

And how am I supposed to know how much distance there normally is to the wall?

You can also look that up: my plans are based on 30/35/30... so RBM one meter. On the internet, you read about 20cm.

But that doesn’t change the mishap now. I don’t know how the defect could be fixed—because even a small compensation sum doesn’t make it better.

Woulda, coulda, shoulda. I find it pointless to speculate whether the OP should have realized it earlier. It's too late anyway. However, I also don't see shifting the entire blame onto the general contractor.

Instead of brooding over it, I would address the general contractor directly and look for solutions. Maybe he is willing to offer a discount for a correction since he gave poor or no advice at this point. Maybe it's possible to swap the toilet and bidet? Then, as a right-hander, at least you'd be fully "in business" again on the toilet. That would be the more important thing for me. ;)

Maybe it’s possible to swap the toilet and bidet?

Only possible if you tear the wall back to the shell construction. Different in-wall frames, different water pipes, different waste pipes – nothing big fits through the bidet ;-) The child has actually and definitively fallen into the well. €150 compensation for the lesson learned. The next house will be better then.