[csharp] Ensure unique property names

[devhl-labs]

Partial fix for #21585

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[wing328]

we suggest users to use name mapping options: https://github.com/openapitools/openapi-generator/blob/master/docs/customization.md#name-mapping

to handle duplicated names in the output

(we tried adding a numeric suffix before and it turned out no one wants to use a model with properties id1, id2, id3, etc even though it compiles)

[devhl-labs]

Why not both? Get it to compile and when they don't like the suffix then use the map. I will concede that numbering could be inconsistent if the yaml or java change, but it should definitely compile one way or another.

[wing328]

never said we shouldn't do both or not adding X different ways to handle such case

just sharing what we went through and the feedback from users when trying to handle such case