[csharp] Fix response data type

[devhl-labs]

Fixes #21686

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[Mattias-Sehlstedt]

Is this the same issue as #21600 or are they different? I see that the affected samples are very similar (but not exactly the same).

[devhl-labs]

I believe its the same. Your pr is preferred since it fixes the cause while i am just patching an error that already occured. If your pr works you should be able to remove the patchNestedMaps method