Split MessageExtensions AttachmentLayout (list|grid) from Attachment.Layout (list|carousel)

[Copilot]

Microsoft.Teams.Api.Attachment.Layout was being reused for both Activity/Message attachments (wire values list | carousel) and MessageExtension Result attachments (wire values list | grid). These are two distinct enums on the wire and need two distinct types.

This mirrors the split in teams.ts — the rename attempt in microsoft/teams.ts#381 (merged 2025-10-25) was reverted in microsoft/teams.ts#382 (merged 2025-10-29) in favor of two parallel types. That context was lost in #193 and #488 (and a related rename in #381).

Changes

• New type Libraries/Microsoft.Teams.Api/MessageExtensions/AttachmentLayout.cs — StringEnum with List ("list") and Grid ("grid"), following the existing Attachment.Layout pattern.
• MessageExtensions/Result.cs — AttachmentLayout property retyped from Api.Attachment.Layout? to the new MessageExtensions.AttachmentLayout?.
• Samples/Samples.MessageExtensions/Program.cs — six call sites updated to the new type (required by the property retype).
• Untouched: Attachment.cs (List / Carousel preserved), Messages/Message.cs, Activities/Message/MessageActivity.cs.

// MessageExtensions result — was Api.Attachment.Layout.List
new Microsoft.Teams.Api.MessageExtensions.Result
{
    Type = ResultType.Result,
    AttachmentLayout = Microsoft.Teams.Api.MessageExtensions.AttachmentLayout.List,
    Attachments = [...]
};

// Message activity — unchanged, still uses Attachment.Layout
message.WithAttachmentLayout(Microsoft.Teams.Api.Attachment.Layout.Carousel);

Non-breaking: the new type is additive, and Result.AttachmentLayout assignable values were already constrained to list/grid in practice. JSON wire format is unchanged.