v2.1.0-preview-0006

This preview introduces a new set of core packages:

• Microsoft.Teams.Apps — the new app-hosting surface for building Teams agents, replacing the legacy v2.0.x package.
• Microsoft.Teams.Core v1.0.3 — foundational primitives (activities, models, serialization) shared across the new package set.
• Microsoft.Teams.Apps.BotBuilder v1.0.3 — interop layer for projects bridging the new Apps runtime with existing Bot Builder–based code.

What's Changed

• Add AI/MCP Client example by @MehakBindra in #486
• A2A Sample by @MehakBindra in #490
• Prompt Preview Support (core) follow-up: string overload + JsonContext by @corinagum in #506
• core: add sovereign cloud support via AzureAd configuration by @corinagum in #507
• Core/activity extensions by @MehakBindra in #515
• Improvements to MCP Server Sample by @MehakBindra in #518
• Update README and icon for core packages by @rido-min in #520
• Simplify dependency management for TeamsBotApplication by @MehakBindra in #521
• Update breaking changes doc by @MehakBindra in #523
• Fix GetLoggerFromServices returning NullLogger during DI setup by @rido-min in #526
• feat: SuggestedActionSubmit handler for 2.1 by @singhk97 in #528