Honor nullable reference annotations during generated dependency resolution

[botoddly]

GameKit.DependencyInjection currently generates GetRequiredService<T>() for every constructor and factory dependency except IEnumerable<T>. Consequently, a nullable dependency still behaves as required.

For example, GraphicsPipelineBuilder accepts Window? because it can build pipelines with explicit texture formats in a windowless container. Window is only needed by AddColorFormatFromDisplay(). Its registration therefore needs an otherwise unnecessary provider factory solely to call GetService<Window>():

AddTransient<GraphicsPipelineBuilder>(static provider => new GraphicsPipelineBuilder(
    provider.GetRequiredService<GpuDevice>(),
    provider.GetService<Window>(),
    provider.GetRequiredService<IShaderLoader>()));

The source generator should use nullable reference annotations when resolving generated constructor and factory arguments:

• T and oblivious reference types: GetRequiredService<T>()
• T?: GetService<T>()
• IEnumerable<T>: retain the existing GetServices<T>() behavior

This should apply consistently to generated singleton and transient constructor injection, delegate factories, and instance factory methods.

Acceptance cases:

• An unregistered nullable dependency is passed as null.
• A registered nullable dependency is resolved normally, including from the requesting child provider.
• An unregistered non-nullable dependency still throws.
• GraphicsPipelineBuilder can be registered with AddTransient<GraphicsPipelineBuilder>() without making Window mandatory.