Update quoted replies (next/core)

[corinagum]

next/core port of #389 — quoted replies support adapted to core conventions.

Changes (matching #389)

• Add QuotedReplyEntity with nested QuotedReplyData (MessageId required, SenderId/SenderName/Preview/Time/IsReplyDeleted/ValidatedMessageReference optional); register in EntityList.FromJsonArray() type switch and TeamsActivityJsonContext
• Add GetQuotedMessages() extension on TeamsActivity to read inbound quoted reply entities
• Add AddQuotedReply(messageId, response?) extension on TeamsActivity
• Add WithQuotedReply(messageId, response?) on TeamsActivityBuilder
• Add ReplyAsync() on Context to stamp QuotedReplyEntity + <quoted messageId="..."/> placeholder (auto-quotes inbound message)
• Add QuoteReplyAsync() on Context to quote a specific message by ID
• Remove WithReplyToId() from CoreActivityBuilder and its call in WithConversationReference()
• Remove dead ReplyToId URL append in ConversationClient.SendActivityAsync()
• Mark all quoted reply types and methods as [Experimental("ExperimentalTeamsQuotedReplies")]
• Add entity, extension, and builder unit tests
• Add QuotedReplies sample exercising all APIs

Deviations from #389 (next/core conventions)

• Entity pattern: QuotedReplyEntity uses Properties dictionary backing store (not direct JSON properties) — matches MentionEntity, ClientInfoEntity
• Extensions vs methods: GetQuotedMessages() and AddQuotedReply() are extension methods on TeamsActivity (not instance methods on MessageActivity) — matches GetMentions(), AddMention() pattern
• Builder: WithQuotedReply() added to TeamsActivityBuilder (main has AddQuotedReply() on MessageActivity directly)
• Context methods: ReplyAsync()/QuoteReplyAsync() (new methods, main modifies existing Reply())
• WithReplyToId() removal: Broader scope in next/core — also removed dead URL construction in ConversationClient and the WithConversationReference() call
• No ToQuoteReply() deprecation: Method never existed in next/core
• No ;messageid= stripping removal: Pattern never existed in next/core
• Rebase() calls: Required after entity mutation to sync EntityList back to base JsonArray
• Source-gen registration: QuotedReplyEntity and QuotedReplyData registered in TeamsActivityJsonContext
• Project-level NoWarn: ExperimentalTeamsQuotedReplies suppressed in Apps .csproj (required for source-generated JSON code)

Test plan

• All 303 unit tests pass (Core: 115, Apps: 149, Compat: 39)
• E2E tested in Teams

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