[csharp][generichost] Make host configuration optional

[devhl-labs]

Library authors can provide a default configuration using the partial method, so we can let this parameter be null.

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Summary by cubic

Make HostConfiguration optional in generated csharp generichost service registration by updating IServiceCollectionExtensions.Add{Api} to accept an optional options callback and use defaults when none is provided. This simplifies the API surface and keeps existing calls working.

• Refactors
  • Consolidated to a single method: Add{Api}(IServiceCollection services, Action<HostConfiguration>? options = null); invokes options?.Invoke(config).
  • Removed the parameterless overload previously generated when no auth methods were present.
  • Applies nullable annotation when NRT is enabled; regenerated samples to match.

^{Written for commit b040be2. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 51 files

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