Add endpoint content type metadata

[ejsmith]

Adds content-type metadata overrides for generated endpoints so migrations can preserve request and response media type declarations.

Summary:

• Add AcceptsContentTypes and ProducesContentTypes to HandlerEndpointAttribute.
• Carry content-type metadata through endpoint analysis and generated Minimal API metadata.
• Emit .Accepts<T>() for body-bound endpoints.
• Emit content-type arguments for generated success response metadata, using .Produces<T>() for typed responses and .Produces() for Result<FileResult> file downloads.
• Cover class defaults, method overrides, multiple content types, and file response content types in endpoint generation tests.
• Document the new endpoint properties and the file download use case.

Validation:

• dotnet build Foundatio.Mediator.slnx
• dotnet test Foundatio.Mediator.slnx