[csharp][generichost] Better handling of duplicate operation ids

[devhl-labs]

Reverts a part of #19715 and handles the duplicate operation id better.

Fixes #19686
Fixes #19865

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[wing328]

can you please resolve the merge conflicts when you've time? then i'll merge and also later add a test to cover this via a separate PR

[devhl-labs]

done

[wing328]

FYI: merged #20070 to add testS covering duplicated operationId moving forward