csharp.sh: add post-generation guidance for dotnet build step

[Copilot]

Unlike other language generators, csharp.sh only produces swagger.json — actual C# code generation requires running dotnet build in the C# client repo. This left users confused when only swagger.json and swagger.json.unprocessed appeared in the output directory.

Changes

• openapi/csharp.sh: Adds a post-run message explaining the two-step nature of C# generation, including the exact commands to copy the generated swagger.json into the C# client repo and trigger code generation via dotnet build:

NOTE: Unlike other generators, the C# client does not generate code directly.
The swagger.json has been placed in: <OUTPUT_DIR>

To generate the C# client code, clone the C# client repository and run dotnet build:
  git clone https://github.com/kubernetes-client/csharp
  cp <OUTPUT_DIR>/swagger.json csharp/src/KubernetesClient/swagger.json
  cd csharp
  dotnet build

For more details, see: https://github.com/kubernetes-client/csharp/tree/master/gen/LibKubernetesGenerator

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[brendandburns]

/lgtm
/approve

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