[rust-server] can't handle duplicate response descriptions

[bjgill]

Description

See swagger-api/swagger-codegen#8169 (comment). rust-server currently has a heuristic where if a response description is provided, we use that to construct the name of the enum variant for that response. This clearly fails when we have multiple responses with the same description, however.

I'm not sure there is going to be a good way to solve this. Perhaps we could detect this particular failure case and bail out of the heuristic? As I remember, the fallback is to use e.g. Response_201 instead, which is resilient to this problem.

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