Activity specific builders for outgoing activities

[MehakBindra]

What changed: Introduced type-specific activity builders alongside the existing TeamsActivityBuilder.
New types

• TeamsActivityBuilder<TActivity, TBuilder> — abstract generic base that all Teams builders inherit from. Holds shared methods: WithChannelData, WithEntities, AddEntity, WithAttachments, AddAttachment, AddAdaptiveCardAttachment, and the Teams-specific overrides for SetConversation/SetFrom/SetRecipient.
• MessageActivityBuilder — concrete builder for MessageActivity with typed setters for WithText, WithSuggestedActions, and AddMention (reads/writes activity.Text directly).
• StreamingActivityBuilder — concrete builder for StreamingActivity with typed WithText.
• TeamsActivityBuilder — now just a thin concrete class with Build() only, backwards-compatible for non-message use cases.

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Why

• Previously TeamsActivityBuilder built all activity types but returned TeamsActivity. So you could do WithText("...") but not access activity.Text after building. This is a problem. Even if we internally cast, the user cannot access activity.Text without casting again
  With dedicated builders, MessageActivity.CreateBuilder().WithText(...).Build() returns a MessageActivity — activity.Text just works. Same for SuggestedActions, TextFormat, etc.
• Since we have limited outgoing activities, this is a clean approach. Developers should directly be accessing Message Activity, and Teams Activity is more of an internal class to coordinate shared properties across outgoing/incoming activities and a bridge to core activity.

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Ripple changes

• CoreActivityBuilder.WithConversationReference parameter widened from TActivity to CoreActivity so any builder can accept any activity as a conversation reference source.
• Context.cs and all samples updated to use MessageActivity.CreateBuilder() where they were building message replies.
• Test suite split into TeamsActivityBuilderTests, MessageActivityBuilderTests, StreamingActivityBuilderTests to match the new structure.

[singhk97]

Looks good to me

[singhk97]

How does this recursive TBuilder definition work?

[rido-min]

it's called CRTP pattern , or in our specific case Fluent builder inheritance or self-referential generics

it's a bit of a workaround for the lack of a Self type

if you are interested here it is a very related discussion dotnet/csharplang#7325

[rido-min]

How can we hide WithType to avoid this usage?

var msg = MessageActivity.CreateBuilder()
  .WithType("not-message")
  .Build()

Eg:

1. Make CoreMessageBuilder.WithType virtual
2. Override in MessageActivityBuilder and check the valid values for "type" to throw in the other cases.