Add protected internal support to CodeGenVisibility for StreamingResponseUpdate

[Copilot]

The codegen customization pipeline could only express internal and public, which left StreamingResponseUpdate stuck with a generated private protected two-parameter constructor. This change extends the existing CodeGenVisibility path so constructor visibility can be customized as protected internal and applies it to StreamingResponseUpdate.

• CodeGenVisibility contract

  • Add ProtectedInternal to the shared CodeGenVisibility enum used by customization partials.

• Generator visibility handling

  • Extend VisibilityVisitor to translate the new enum value into MethodSignatureModifiers.Protected | MethodSignatureModifiers.Internal.
  • Update modifier assignment to clear and replace all visibility flags consistently for constructors, methods, and properties.

• StreamingResponseUpdate customization

  • Add a CodeGenVisibility attribute on the StreamingResponseUpdate customization partial targeting:
    • StreamingResponseUpdateKind kind
    • int sequenceNumber

• Regenerated output

  • Regenerate StreamingResponseUpdate so the generated constructor is emitted as protected internal.
  • Refresh the exported OpenAI.Responses API surface to reflect the new constructor visibility.

• Regression coverage

  • Add a focused generator test covering constructor visibility remapping through VisibilityVisitor.

Example customization:

[CodeGenVisibility(
    nameof(StreamingResponseUpdate),
    CodeGenVisibility.ProtectedInternal,
    typeof(StreamingResponseUpdateKind),
    typeof(int))]
public partial class StreamingResponseUpdate
{
}