[csharp][generichost] Add partial methods to host configuration

[devhl-labs]

Please merge #24342 first

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Summary by cubic

Add partial hooks to HostConfiguration in csharp generichost templates and simplify IHostBuilder.Configure{{apiName}} by removing the IServiceCollection parameter and making options optional. Updates all samples to the new API.

• New Features

  • HostConfiguration is now partial and exposes:
    • partial void OnHostConfigurationCreated() for post-constructor setup.
    • partial void OnAdd{{apiName}}HttpClientBuilder(IHttpClientBuilder builder) to set default HttpClient config before user overrides.
    • partial void OnServicesAdded(IServiceCollection services) (invoked via NotifyServicesAdded) after DI registration.
  • Configure{{apiName}}(this IHostBuilder, Action<HostBuilderContext, HostConfiguration> options = null) replaces the old (context, services, options) signature; options is now optional.

• Migration

  • Update calls to .Configure{{apiName}}((context, options) => { ... }) or omit options entirely.
  • If you previously used services inside the Configure lambda, move registrations to builder.ConfigureServices(...) or use the new partial hooks.

^{Written for commit 74aa879. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 303 files

<sub>Note: This PR contains a large number of files. cubic only reviews up to 200 files per PR, so some files may not have been reviewed. cubic prioritizes the most important files to review.

Re-trigger cubic</sub>