fix(csharp): Improve gRPC code generation

[amckinney]

Description

This revisits the gRPC C# code generation and updates it to match the latest changes in the C# generator. This includes:

1. Updated enum representation and .ToProto + .FromProto mappers.
2. Fix an issue with rendering the gRPC client reference.
3. Update protoc-gen-openapi to unwrap synthetic oneof types (re: https://github.com/fern-api/protoc-gen-openapi/releases/tag/v0.1.7)

This also updates seed so that we preserve the S3 upload/download semantics required for gRPC SDKs. With this, we include the input proto specs alongside the generated C# SDK so that we can lean on the .NET runtime generation required by the gRPC library.

[github-actions]

🌱 Seed Test Selector

Select languages to run seed tests for:

• Python
• TypeScript
• Java
• Go
• Ruby
• C#
• PHP
• Swift
• Rust
• OpenAPI
• Postman

---

How to use: Click the ⋯ menu above → "Edit" → check the boxes you want → click "Update comment". Tests will run automatically and snapshots will be committed to this PR.